The top and bottom NFL teams aren't that far apart, if that were true we'd have some team lose every game every year, or have several teams win only 2 or 3 wins. Last year half the league's teams were .500 or above, several more were just under that.
Sprinkle in a few key injuries and a "top" team loses to a Lions-type team.
Of course you have teams like the Jets who flop every year, but (and PLEASE don't take this as a statement of fact) who knows, for some reason they may not have the burning desire to win SBs...