Is there parity in the NFL?

Parity simply means every team has a chance to win. Doesn't mean every team will take advantage of that chance. In addition, the divisional set-up throws off the numbers. Each team has a 1 in 4 chance of winning their division, but of the 24 non-division winners, only four make the playoffs (a 1 in 6 chance). So if one team has a strong, multi-year run within their own division (Indianapolis, Pittsburgh, New England) the other teams in those divisions will consistently have their playoff chances reduced every year, which explains Buffalo and Cleveland.