I'm sure somebody more knowledgable than myself could comment further but I'm pretty sure that Easy's 319's are not all EXACTLY the same - I believe some have a higher gross weight limit or something and typically tend to be used on longer routes I believe? The A320/21's they have were 'inherited' from GB Airways I think - not sure if EasyJet would actually have bought them if that hadn't occurred and would surely have preferred greater fleet commonality? (ignoring the common type rating aspect and focusing more on the different configs/fuel burns etc.)
Same story with FR too; some of their 800's have higher weight limits than others (66.9 to 74.9 I believe?) Memory fails me as to what that weight is, Max TakeOff Weight I think...?
At the risk of slight thread drift isn't that 3 crew requirement thing a bit of an issue with the 737-900 too? With 4 crew the max pax capacity on a 73 is 189, the 737-900 surely has room for more seats but would need an extra crew member... Does the 900 have something else going for it?! Surely the cost of an extra crew member would not be made up by the (assumingly small) number of extra seats?