I'd suggest that the reason QF Link Q400's have 72 seats is because of the Oz F/A to Seats ratio (1:37). If 78 seats were fitted then 3 F/A's would be required which would make it uneconomical. The NZ rule, I believe, is 1 F/A to 50 pax (not seats) which means that 2 F/A's can crew the 78 seat configuration.