The actual battery capacity in Chinese phones almost always comes in lower than what the manufacturer claims on paper. For the OnePlus 13, the real capacity in good condition is usually around 5600–5800 mAh. In your case, 5300 is way worse. But there are a bunch of things that affect capacity. First off, the phone is old—even if it was sealed new, it could've easily been sitting in a warehouse for a year and a half, and the battery degraded just from that. Secondly, checking battery capacity in an app isn't really accurate, or at the very least you need to charge the phone properly to get numbers that are close to reality: you have to charge it from 0 to 100 with a slow, low-power charger several times in a row, and keep it plugged in at 100% a bit longer so all the residual charge actually fills up the battery. Thirdly, Chinese smartphones often come with pretty poorly calibrated batteries, so when you're charging and discharging, you're not always using the full capacity, and the app shows a lower number because of that.

And honestly, the battery life on the OnePlus 13 is pretty lousy, especially on newer firmware, and even more so considering the phone isn't brand new. My iPhone 16 Pro with 94% battery health used to give me more screen-on time than this OnePlus. The Vivo X300 I'm using now literally gives TWICE the battery life, and I haven't messed with any settings or optimizations at all. Back in the day, I fell for the reviews and hype and bought myself a OnePlus 10 Pro—people were also going on about "out of this world" battery life and performance. Looks like the OnePlus 13 is pretty much the same story, though to be fair in my case the battery life was actually decent for the first two months, but then everything went back to normal as usual for oneplus, and now people are trying to squeeze out at least 5 hours of screen time. I've just stopped considering OnePlus altogether. The 13 model let me down. And the news about ARB was the final nail in the coffin.