Geographic Mapping

I installed a new mapping feature on my blog. It's over on the right hand side, down near the bottom. It's shows where people are located when they read the blog. Well, it actually shows where their IP address is registered. You can see Zel checked it out from the UK! Can anyone explain why I have that big bubble in the Midwest? Is it possible that all Comcast IPs are registered to a town out there, or something like that?