Video quality while on 3G

The same day I got the iPad 3G, I noticed a very contrasting video quality while using Wi-Fi and while using 3G. Videos watched on Youtube would have HD quality (Wi-Fi); same videos, while on 3G, would show horribly. Yet, videos on Apple, while on 3G or Wi-Fi, would show perfectly. All that whether using the app or the web browser, tested both.

Is it Youtube serving a very low quality video to 3G users? Is it AT&T doing it? Is it something built-in on iOS?