There could be a few things at play:
1) You did not say if the website you visited required you to 'log in'. If you did then the system is tracking based upon user names and probably not a cookie of some sort.
2) With regard to your IP address, you did not say how you obtained this. For example, if I issue the following commands from a command prompt on my work pc: ipconfig -release then ipconfig -renew I might very well wind up with a different internal IP address, however, my external IP address has not changed. The same secnario holds true if I issue the commands from my home PC since it sits behind a router that is attached to my cable modem. In both cases your internal IP address has changed, your external has not.
I will have to say that one of the aforementioned conditions are true because, typically, a cookie created in IE can not be read by FireFox or vice versa. Further, when you create a cookie, the browser decides where it is placed on the client machine and, as far as I know, this can not be overriden.
Lastly, as you have said, there is no "full proof" poll/survey system and I think you just need to accept it. For example, if you look at www.cnn.com
they have a poll on their homepage everyday with a little disclaimer that says "Polls are not scientific" which is a nod to them knowing there is no full proof way to prevent ballot stuffing and the like.
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