Apple is changing the battery calibration for older Apple Watch models, which the company has now announced in the release notes.
After updating to watchOS 9, your Apple Watch Series 4 or Series 5 will recalibrate and then more accurately estimate its maximum battery capacity.
Battery calibration – no changes in the test yet
Nothing can currently be determined in our test, but it may well be that it takes a while before the watch recalibrates and the battery indicator behaves differently.
Cover via iFixit