Parity, atleast as far as potential to acquire talent, exists in this league.
Every team faces the same cap, gets to draft, gets a crack at FAs, etc.
Parity does not, however, exist in the two most crucial fields:
Talent evaluation and coaching.
The IND, SD, NE, PHI, and SEA's of the world are prennial contenders because they've got good talent evaluators (which has allowed them to draft blue chip guys and quality backups) and excellent coaching (not to mention, stable coaching).
On the flipside, you've got the OAK, ARI, WAS, HOU, and DET's of the league which suffer from chronicly bad talent evaluation (or a lack of picks to acquire talent such as WAS) and have a total lack of stability on the coaching staffs.
And in the middle of the league, you've got the teams in the middle of the pack. The teams with average talent acquisition and some level of coaching but certainly not on par with the top tier. This tier can be de-railed by injuries (CAR), can be sunk by poor coaching (NYG, ATL) or can catch lightning in a bottle (TEN, NO) on a result of an excellent pickup for their roster (V. Young, Brees, R. Bush) and some luck regarding injuries.
Each year you get a couple middle tier teams that make a temporary jump. Rarely are they able to maintain their elite status (it will be fascinating to see what NO can do next year)-but if the talent acquired is good enough, then it is possible for them to jump to the elite tier.
The point though is this. While any team can beat any other team "any given sunday" as they say, if you compare the body of work over the course of a season and then compare trends between seasons it becomes clear where a team falls. Sure Houston might beat Indianapolis one week....but there's no doubt that Indy is the superior franchise at this moment.
That's how I've been approaching the last few seasons.