The percentage of the battery is an estimate. After some updates, they may possibly be thrown off. I always start a little after an update. I can be completely wrong and that's fine. Anyway, a restart doesn't hurt, and sometimes technology also needs a nap.
Updates can throttle battery performance, usually older models that run new updates optimized for newer devices. Apple was called out for a while back, but it wasn't malicious for the purpose. However, an update to an S4 does not seem to have that problem, as it is still the latest model to look up. Notice how someone reports some issues by updating an S2, for example. This doesn't seem to be the case here, but I figured I'd mention it.