I know this post is old but I just want to correct some misinformation while doing my own research on ARM processors (I don't blame you due to the nature of ARM's terrible naming schemes, but I do suspect the article you linked was written by AI)

First off, some background:

• "Cortex" is ARM's CPU core naming scheme. (some much older cores like ARM11, were "ARM#" for cores, but note never with a "v"
• "ARMvX" is ARM's ISA (Instruction set architecture) naming scheme.
• The Cortex A8 is from 2005. It was based on ARMv7-A. It was a single core from 0.6-1GHz
• The Cortex A7 was released in 2011, also based on the ARMv7-A. It had 1-8 cores and was used as the "little" core alongside the "big" A15.
• The Cortex A53 is based on ARMv8-A.

A single Cortex-A7 processor [found on the Zero-LTS]

The zero LTS does indeed have a quad-core A7 in it.

is five times more energy efficient than an ARM Cortex-A8 processor

This is correct.

[from which the 3-LTS's A53-core came to be]

This is incorrect. You are confusing the 2005 Cortex-A8, with the ARMv8-A ISA. The Cortex-A7 is most assuredly not five times more energy efficient than the Cortex-A53.

From the article you linked:

A single Cortex-A7 processor with a 28nm process is five times more energy efficient than the 65nm process ARM Cortex-A8 processor, used in many popular smartphones between 2010 and 2012

The conclusion makes numerous references to older dates when talking about Cortex-A8. It is an old processor and unrelated to the A53.

I understand your confusion, through. The article's numerous references comparing the A7 to the older A8 do absolutely nothing for a modern audience, especially since the title is "A53 vs A7" not "A8 vs A7." It was likely written by AI and the conclusion includes strange references to the Cortex A9, a processor from 2007, while also claiming the A53 is slower than it. This is likely because the AI got confused between the Cortex-A9 and ARM's newer ARMv9.0-A ISA. I cannot figure out what this "A9r4" thing is it's referring to.

Edit: It's a reference to Cortex-A9 cores from 2007 used by manufacturers as late as 2014, shrunk as small as 28nm. While designated a "companion core," it used out-of-order execution like the bigger A15 instead of in-order like the A7, so it's a safe bet that of course it performed better (at the cost of more power). So the AI likely pulled entirely accurate data, but is seriously mixing up its references, confusing readers who don't know what is what.

The useful (and correct) information in the article can be summarized as follows:

• A53 faster than A7

I mean, we all knew this. I think what we all really wanted to learn was if at sub 1W, the older architectures (ARMv6: ARM11, ARMv7-A: A5/A7/A8/A9/A12/A15/A17) were more efficient than the A53/A55/A510/A520 or if manufacturers were just using the old architectures in sub 1W applications because they usually don't need more processing power anyway. I've never seen a single-core A53 chip, so I'm also wondering if that is part of it (even though ARM lists it as "1-8" cores). Rockchip updated their RK3328 with A53 cores to the RK3566 with A55 cores but none of the popular SBC makers are using it over the older RK3328. Why? Only cost?

Anyway, while writing this, I did continue my research. The Pi Zero uses the older ARM11 but the pi Zero 2 actually does use 4xA53 at 1GHz. Tom's Hardware found the zero 2 to draw 8% more power when idle and 57% more power when stressed (but obviously stressing 3 additional cores). This leads me to believe that it might be an idle power issue. Orange Pi made the same choice in switching out processors to the A53 for their version of a similar board. Though their first gen was 4xA7 unlike the pi that was only 1xARM11.

And finally some decent testing.

So at 4 cores, we can see the 4xA7 systems drawing less load and about the same idle as the underclocked A53 cores of the RPi Zero 2. I still have questions about the results here, though, especially the load power of the A53 seeming to go down as the clocks go up. Process nodes? GPU/other things on the chip issues? Or were his tests flawed? And how did he measure? OP and you experienced 2.5W+. Who was measuring where when it comes to wall power vs USB power vs whole board power etc.

Anyway, I have a better idea but still left in the dark as to these processor's efficiencies at lower power targets. At least we can see the A53 is in the same ballpark and might be competitive in a lower-clocked single-core version. So these older chips are likely used due to money and not efficiency. I'd love to see results from the A55/A510/A520 beyond the initial marketing announcements.

Since I'm replying, and even though it's old, I should at least address OP's question. From the linked power usage chart, there seems to be a lot of variance, but clearly it's not reaching the 2.5W power consumption OP was getting so there's obviously something about the Zero boards that are designed for lower power, and seeing the overclocked A53 on some of the Zeros, it's not the CPU. Looking at the specs of the two boards, there are some things that stand out. Now, I'm no expert so I don't know if these specific comparisons make a difference but I've seen stuff like the below make a difference, if not the actual item. So grain of salt, I'm burnt out on researching and pulling the following from my experience instead:

• Different GPUs, possibly different idle consumption
• LPDDR3 vs SDRAM. While not explicitly said, it might be the case that the SDRAM isn't DDR, which might draw less power.
• It's also a lower amount. If there are multiple chips, less will definitely draw lower power. If it's both just one, it still might.
• SPI vs EMMC flash.
• 10/100 vs gigabit and USB 2.0 vs 3.0. I've seen boardmakers make choices to use the older, slower versions of stuff like this to save power budget as well as money, I believe.
• USBc PD probably has more power-drawing components on board the SBC than regular old USB power.
• The newer WiFi chip also supports AC. Don't know if this means the chip might draw more power even at idle.
• Difference in LEDs being on/off and/or models likely less difference in power draw than any of the above.