Because I suspect they check whether you have already voted by some portion of your IP address, and if you use a common ISP-wide proxy (that is provided by your ISP), it doesn't register after the first person that votes.

On top of that, the top100 voting site serves you different ads, and even the website itself is in a different language depending on what web proxy you use (i.e dependent on the web proxy's location). For example, the voting site would show me German text if I used a web proxy located in Germany, and the way the voting works is 2-steps (captcha on page 1, then copy a text box filled with random letters into another textbox on page 2 to verify.), while if I voted without a proxy, it would be in English and wouldn't have the page 2 verification.

So very clearly, they already do different things based on where you connect from, and my suspicion is they use your IP address is various annoying ways to determine if you are double voting.