Could you imagine storing each users details in the database for each poll they vote in, the forum would crawl along and be totally full of useless data.
It would mean storing each uses ID each time they vote in a poll, have 1000 users vote in a poll it would mean storing 1000 ID numbers for that poll in the database, thats quite an overhead.
You will only have the problem when you delete a poll is you start with a fresh database again, in normal use each poll with have different ID number so the problem woouldn't arise.