This was actually mentioned on a local radio show recently, comparing the SEC and the B1G. For the SEC, when one of them wins, they all win. "Great victory for the SEC" and all that. On the other hand, if the B1G loses, it is a condemnation of the conference as a whole. Obviously that is referring to the national media perception.
^^^THIS^^^
I realize that it's the media that tends to drive it, but fans of SEC teams have bought into it.
When USC was having it's run during the Pete Carroll era, all Trojan fans heard was "The PAC is weak", "The PAC is USC and the 9 dwarves", etc., etc. When we would lose to a PAC team, then it was proof that USC was "overrated". Yet, it was only PAC teams that could beat us.
With the SEC, if a top tier SEC loses to another SEC team, then it's proof of how tough the conference is, rather than proof that the losing team was overrated.
Excuses are made for SEC teams that lose bowl games too. It's always "they didn't want to be there", "they were disappointed that they weren't in the NCG". I don't see those same excuses made for teams from other conferences.