Your browser has a “fingerprint” which includes your browser and operating system, what it identifies as its accept headers (EG compression support), your plugins, your time zone, screen resolution and color depth, your system fonts, whether or not you disabled cookies and so forth.
So perhaps it is for this reason that Google and Microsoft gladly welcomed do-not-track and why Microsoft promised not to scroogle you or track you on Bing – because thanks to Big Data they, Google and certainly the feds, don’t have to track your clicks to external sites using conventional methods like URL redirecting, thanks to this fingerprint you broadcast. Maybe you can figure out how to randomize what your browser reports on top of Tor and virtual machines, but that’s a lot of effort and knowledge pretty much everybody either doesn’t have or is not inclined to use.
Think you have a common setup and can blend in with the crowd? The EFF set up a page right here where it takes the data I mentioned, but without taking your IP into account, and compares it to the arrays of data that other people who visited the page supplied (currently 3.7 million). It says my browser supplies 21.81 bits of identifying information (and breaks down what they are exactly), and those bits I gave are unique enough to completely segregate my desktop and my phone from the 3,668,022 people who had visited this site before me. One in 3.7 million, I guess I am special after all.
You can’t hide bro, certainly not if you’re aiming for 100% confidence. Not even with Tor – which, on top of its steady stream of vulnerabilities and many nodes running weak encryption, if you use Tor or PGP, the NSA can legally tap you according to a FISA court ruling. Eric Schmidt was right when he advised that you abstain from doing whatever it is that makes you want privacy. So you can’t hide; though if you want to keep a low profile, my advice, just try to blend in and find something else to be so passionate about.