This is one of those things where over time, these might not match and we need to get some cleanup tools available.
On the Group Management screen, it just checks to see how many user_id's are in that group.
When you click on that number, it actually checks to see how many are in that group, and have entries in the users, user_profile and user_data table.
With some of the various bugs we've had with deleting users, moving users to other groups, etc since 7.0 came out, this is why these numbers might not quite match.