Maybe
I should have been clearer!
The reason the CAA allow the easyJet A319 to operate with three cabin crew (when the seat blocker is fitted), even though it is certified for 156 seats is because the same type is
also certified with 150 seats. It is a peculiarity of the
easyJet ordered A319s, not a general policy to allow a higher than 1:50 ratio of cabin crew:seats.