> but most phones are pretty good at that afaik

I clearly haven't had good luck on this front. My iPhone is showing a battery health of "service", and a maximum capacity of 77%, after just 357 cycles.

You could try draining it to zero and recharging it without using it. Sometimes the battery life calibration gets a little off and reports bad numbers. There's not a perfect way to monitor battery health a lot of it is based on the voltage curve of the battery and that's somewhat variable from battery to battery in a way that can mess with the battery health estimate.

There's also the chance you get a slightly bad battery and just got screwed on the lottery.