You are likely doing nothing wrong. This appears to be (another) Google bug.
The Google Play Store system seems to be broken in yet another way (it also thinks updates are available for apps when there is no compatible update available).
Release notes for all apps seem to be broken in the Google Play Store.
A discussion on XDA Developers Forums indicates others having problems with Google's Play Store.
Similarly, a discussion on Reddit talks about people having the problem with Google Play Store that you identified. A member of that discussion postulates a hypothesis as to why this may be happening:
Google strongly encourages turning on automatic updates and just blindly accepting every update. Sure seems like they've decided that if they blind us to what the updates contain, we'll just turn on automatic updates since we can't make informed decisions about them without the update info.
From the data I have gathered, this Google bug seems to be affecting all versions of Android.