The idea is not that your resolution is unique it's that the maximized window size is unique. They can then track that you have more than one monitor, what size that monitor is, and, probably with some easy math, a jist of how many you have. That setup is probably a lot more unique than expected. That plus possibly some canvas finger printing or something similar could probably keep pretty good tabs on you. I have 3 identical 21 inch monitors and I'm browsing on the left-most monitor. Anyone else have that same setup?