在 reddit 的這篇文章中提到
When charging, the watch will stick at 70%. This is because we have no resolution between 70% and 100%, so attempting to provide numbers was misleading at best.
For developers, the best practice is to not attempt to provide a numeric value while charging, and instead just show an image (the system charging animation and settings menu both do this while charging). You can usefully show numbers after unplugging the watch, though. Attempting to provide numbers while charging will lead to user confusion.
This is an unfortunate but unavoidable feature of our hardware.
簡單地說就是由於硬體因素，Pebble Time 在充電時無法取得 70% ~ 100% 的正確電量
這篇留言有 @pebblehelp 的官方帳號回覆，所以應該就是這個原因了