This happens mostly when you are inside a network. We can only collect public IP address and in that case all the systems inside the network will have same IP. Also sometimes Internet Service Providers suffle the IP addresses. So in that case your IP might have registered a voting.
If you are allowing only registered users to vote, then I will suggest you to go for the PRO version (link available on description page http://wordpress.org/extend/plugins/wti-like-post/) which allows you to cross check voting with user id. So you won't have this problem with IP.
Thanks