Contrary to popular belief airlines do not have a fixed pitch for all their seats, some of those quoted here are averages, others are minima. Even in one aircraft there can be different pitches in different places due to the spacing of bulkheads, doors, etc that the seats must be fitted in between. And there are usually noticeable differences between aircraft types.

Then there are airlines where even on one fleet the seats are laid out differently. This is most noticeable where operating secondhand aircraft (it is surprisingly difficult and expensive to rearrange seating once installed). Finnair for example has 6 MD-11s in their long haul fleet, all done differently. The original pair are quite OK, a recent acquisition is a real dog for pitch. But they look identical from the outside !