I'm led to believe that any modern device's battery doesn't really have a fixed, concrete "charge level" but rather a complex set of factors to measure and estimate in software to come up with the oversimplified number we all like to have. And I think the Playdate lacks a dedicated battery measurement chip, making that even more true. (Correct me or add detail if you know more!)
So the software-displayed NUMBER could be a pretty separate issue from the charge itself and resulting play time.
I have one of the early Playdates and I've seen the charge go up or down quickly after plugging/unplugging cables—or even exiting an app to the launcher. Or at least I saw that a year ago... I kind of stopped paying attention!
Maybe the software for estimating that % number just needs more refinement? (Either way, my Playdate lasts a long while so I'm happy!)