'Last Modified By' Column

Hello, 

I am working in a P6 v8.1 schedule with multiple users.  I noticed some Actual dates that looked a little suspicious to me in my portion of the schedule, so I put up the 'Last Modified By' column to see who the last person was to modify it. I asked that person to please stay out of my portion of the schedule and was met with confusion.  So I looked to see what activities I had last modified and was surprised to see activities showing my name that I am 100% sure I have never touched.  Nobody else has my password, and the activities were not added by me.

I'm wondering if anyone else has experienced this.  Is the 'Last Modified By' column totally worthless then?  Is there something that could be causing this error and is there any way to fix it?  

Thanks,

Nicole