Its unlikely they are looking at webpages and whether they were mobile format or not. The data packets are tagged as tethered, it doesn't matter whether you were viewing mobile pages or not (and that isn't going to conceal the tethering). At least that what folks here that are smarter than me have stated. If true, that means that AT&T can detect any tethering, they just choose not to enforce with zero tolerance (possibly because its simply not practical), but rather to use some additional parameters to decide whether a person is going to receive the "warning" email and subsequently have a tethering plan added to their account.
Nobody here knows for sure what parameters AT&T uses to trigger the "warning" email (and probably not info that AT&T is going to volunteer). But volume of tethered data has been reasonably speculated. I'm surprised to hear that you were able stream YouTube, other movies, and downloads, without getting caught. Could be you were just lucky; or you are in fact correct that tethered data volume is not the key indicator. Nobody except folks at AT&T know for sure.
My advice is still to keep tethered usage to a minimum, in order to stay of their radar.
Sent from my vR:M8