The NFL is notorious for its promotion of parity. With only 16 — soon to be 17 — games on the schedule and the fickle nature of the game, the idea is that any team, like the San Francisco 49ers last year, can make the jump from bottom-dweller to top of the world.
One of the primary reasons this happens is regression.
A dominant football season is like catching lightning in a jar. When it happens, you need to cherish it and use it as much as you can. Because the fact of the matter is most teams go through some form of regression from their peak years.