Dynamic IP addresses and use of the same IP address by multiple people.
All it takes is a couple of friends, acquaintances or others accessing using your home network and it'll confuse the hell out of the stats. And that's without going into IP ranges for universities, schools, offices large and small, and your local coffee shop.
Then add in IP address pooling by ISPs, where every time a user connects (or every week, month or year) they're issued a new IP, and you end up with an unclear situation.
I won't start on how cell/mobile phone networks further confuse the situation ;)