Actually, "topping off" is a thing in iOS, and macOS for that matter (I would assume watchOS as well). Apple battery percentage is not linear in the traditional sense. That is why iOS reports 100% longer than it actually is. This has been discussed about at length and can easily be proven by Coconut Battery.
When iOS reports 100%, its actually anywhere from 95->100%. Therefore, the moment your device says 100%, its actually roughly 95%. Only after another 20 minutes of charging does it reach true 100% status. iOS/macOS Battery percentage is as followed:
100% Reported = 95-100% Actual
90% Report = 85.5% Actual
50% Reported = 47.5% Actual
10% Reported = 9.5% Actual
As the battery drains, the difference between reported and actual closes. But because the first 5% is skewed, it causes the rest of it to skew as well. Only after 95% does it become "linear".