Here is a weird problem. My moderator (I don't know if her rank is relevant, but putting all info in) is the top poster. However, her nickname displayed on the 'Most Active' widget isn't the nickname she uses on the site.
In the dashboard, it shows the top commenters and her name is accurate. However, on the widget it's wrong. The name is not in her profile at all so that had me do a little digging and it appears it was a name she used many years ago on the site.
I asked her what she did to her account that changed her name like that and I will update this when I find out, but for now, this is still a bug regardless. Under 'Active Users and Guests' in the dashboard, it shows the correct nickname. That is the name that should be displayed under the widget regardless of what she did to her account in the past.