Hey SpK, I read your msg and there's something you could try, just for testing.
There are several ways to fingerprint your own browsers and also your machine in a pretty reliable unique way. One is the CL way for instance that involves detecting the plugins you have installed in your browser or, in other words, the extended capabilities your browsers on your machine advertise to the website.
Let's say for the sake of clarity you'll have a certain version of flash player, shockwave player, java jre, maybe qt or silverlight applets, maybe other plugins that a simple js snippet is able to detect easily. It's reasonably foolproof for the website to fingerprint not only your browser/s, but with a bit of statistics even your machine altogether (at least before you upgrade ALL your plugins for all the browsers on your PC). And all this even without the need of cookies (tracking, ever-cookies, zombie-cookies) or checking the IP. The combination of all these version numbers might suffice to give the website a statistically reliable grasp on whether you're a returning visitor or a complete newcomer.
It's also possible that if you use a vb.net html control you keep advertising yourself as the same IE browser with the same plugins installed and that *might* be the cause. Maybe giving it a try with firefox switching on and off the various plugins (not the addons the actual plugins) might be a valuable test bed for your investigation on YT.
Also I'm blind guessing a lot of stuff here, so I genuinely hope it helps.