I think the 30, 31, 32, 33 inch seat pitch happens because of the way the cabin lays out. Bulkheads, exit rows, doors and galleys must all be worked around when the seat layout is installed. This might lead to a non-perfect fit. If there were 10 inches left at the back of a cabin you couldn't fit an extra row of seats in but you could put an extra inch of seat pitch onto 10 rows. Things might change depending on how seats align up with windows and fixing points and things.

I guess it's just the way they fit.