Did you copy and paste the URL? It adds an id on the end, it doesn't track me when I remove the id.

You sure it doesn't make you just think that it recognised you? Do you have all the same ID and the "achievements" in both? I started in Safari, switched to Chrome and the script said "Welcome Back" although everything looked like the script made a new profile.

I would guess IP Address is there anything else?

Probably the URL's identifier added on the path...