I go to site settings and People and Groups, click on Groups on the quick launch to view all user groups.
In the list of groups, there are more than a few user accounts that are displayed and I'm not sure why. I've deleted a user account from the site collection and added them back in to a group, and that account will appear again in the group list.
My team member who's in charge of managing accounts noticed this and said she had added 150 or users to different groups, and randomly 20 or so of them would appear in the group list.
Here is a partial screenshot of what I'm talking about. The top three are groups, bottom two are user accounts.