question about IP addresses

This might sound bizarre but I'm not trying to convey the whole of the drama here. I just have a story with a technical question attached to it. I hope no one feels reading this is a waste of their time.

I take part in some heated controversial discussion blogs and my mom is into this as well. Both of us have been posting on the blog of a fellow who really doesn't like us or the opinions we espouse. At one point, he suggested we were the same person and that my ma fabricated me, but he ceased that after I offered to meet him in person (we live near the same somewhat large city).

Now his game is that he insisted that when I go to my college, I am posting under my mom's screen name since he's watching the IP addresses.

We've got AT&T DSL but the college I'm reffering to is in a different municipality. I wouldn't think that they would use AT & T.

He's using sight meter.

So is a glitch possible? Or is this user error (because there is no question that I don't post under my mom's screen name and I don't think she's ever set foot on the college and definitely not during the time in question).