2
It's probably by allowing anonymous users to vote.
The problem is that multiple users can share the same IP address (eg in a company or even at your home (when parents and children vote, even with their own PC, they have all the modem's IP address)) or one user can have multiple IP addresses (by reinitialising their modem).
So for anonymous users it is simply not possible to assure that one man is one vote.
But on the other hand, I think the the poll vote function is not
very tight.