If your 14-12 Sun Belt vs. Southland record is accurate, I'd be curious to see how many of those 12 victories are five years old or older.
The Southland victories do indeed tend to be distributed towards older games. But part of that, I think, is that in recent years the match ups have been better for the Sun Belt in that they've been playing weaker Southland teams. There's no doubt that the Sun Belt is tougher than the Southland overall. But the point is that it's not like "another level."
Another thing is that if we are going to compare scores we might as well use the Sagarin Ratings because that's what that system is based on. It's comparing all the scores in comprehensive way. And when you do that this is what you get in terms of conference strength ratings as of yesterday (I say that because yesterday is when I did the math to combine divisions of conferences because Sagarin doesn't report them combined) starting with the weakest Big 5 conference:
Atlantic Coast Conference 75.3
Mid American Conference 59.8
Missouri Valley Football Conference 58.33
Conference USA 55.45
Sun Belt 54.34
Big Sky 48.36
Southern 46.82
Colonial 45.90
The MAC, CUSA, and the Sun Belt are clearly more like top level FCS leagues in caliber than they are like the conferences that form the mainstream of FBS. And it would never be a shock to see even the best teams in any of those leagues as they are now lose to a top FCS team.
Take a look at the Sun Belt in particular since that is what this thread is about. It's rated about 4 points below the strongest FCS conference and about 6 points higher than the second strongest FCS conference. Meanwhile it's rated about 21 points lower than the weakest conference among those 5 that form the mainstream of FBS. It clearly "fits" in better with FCS than it does with real FBS football. There is clearly a much bigger difference between the Sun Belt and the Big 5 conferences than there is between the Sun Belt and FCS. In fact the Sun Belt is right there in the midst of the upper range of FCS conferences.