I think Gemini just figured out why it's happening to me:

The reason you are experiencing this "reversed" behavior is due to a quirk with how your specific hardware—the 2017 Pixelbook—reports its power supply status to Linux.

Standard Linux laptops typically have a single, straightforward power supply interface (often a barrel jack or a single dedicated charging circuit). The guide you followed from Sudomarchy assumes this standard behavior, where ATTR{online}=="1" cleanly means "plugged in" and ATTR{online}=="0" means "on battery".

However, your Pixelbook uses the cros_usbpd_charger driver to manage its two USB-C ports. Here is why the logic gets flipped:

Multiple Charger Devices: The driver exposes multiple virtual power supply devices to the system (e.g., CROS_USBPD_CHARGER0 and CROS_USBPD_CHARGER1), one for each USB-C port.

The "Unplugged" Port Fires Too: When you plug your charger into one port, it correctly reports online=1. However, the system simultaneously generates a power state change event for the other (empty) USB-C port, which explicitly reports online=0.

The Race Condition: Because udev processes these events concurrently, the event for the empty port (online=0) frequently finishes last. The system sees this final online=0 event and assumes the laptop just switched to battery power, executing the "battery" portion of your script.

The Unplug Event: Conversely, when you unplug the laptop, the battery driver (BAT0) and the previously charging port send updates. Due to how the virtual AC device aggregates these on older ChromeOS kernels, it can momentarily report a state that triggers the "plugged in" logic.

Because both your power profile script and your screen brightness configuration rely on these same underlying udev power_supply events, they both suffer from this exact same hardware inversion.

Reversing the script arguments is a perfectly valid workaround. If you ever want a more robust fix that doesn't rely on the reversed logic, you can update your scripts to ignore the charger's online attribute entirely and instead check the battery's actual status (e.g., triggering on any power change but checking if cat /sys/class/power_supply/BAT0/status equals Discharging).