Excel giving wrong user name as having the file open

[mayday1]

We're using Excel 2007. Under Personalize your copy of Microsoft Office, in the User Name box it clearly says Mary B. But when she's in the file and I (or anybody else) tries to open it, it shows that I'm in the file, not Mary.

It's been doing this for days. We've all booted. She's deleted her name and re-added it. What's going on here?