Your user ID number is also stored in the db as the last voter so until someone elase votes in the poll you can't vote again even if you delete your cookies.
The ID numbers of everyone voting isn't stored in the database and kept becuase it would lead to bad performance, imagine a poll with a 1,000 votes you would need to store who voted for each and every one of those 1,000 votes in the database making the database size larger than it need be and also slowing down the performance of the forum.
Edited by -boRg-