It is true that this only resets when a logout happened. So when they keep all their tabs open all the time (which, admittedly, I also do) it could be that it is not updating the way it is supposed or expected to.
I think it is a challenge to measure visit in another way that is as reliable. I also remember some other community where the superusers did not like how this works. I'll share the feedback with the product team.
What you could do: Change the phrase of this button to be more specific what it does: "activity since last login". This will help to manage expectations, unfortunately there is no other way to tweak this.
Another tip: Depending on how smooth your login/logout process is, they could also quickly do these steps to update the list. All other tabs will reload in realtime when a logout/login happens to stay up-to-date with permissions.