The volume of such emails may be different at the 2 airlines, and that might be a factor. Also NZ has aggressively worked at improving inflight service for the past 2 years, with great results. So they are on the lookout for evidence that their service has greatly improved; while CX has pretty much stayed the same (or declined slightly, for various reasons). Kudos to NZ, but it is a much smaller airline than CX (maybe 1/3 the long-haul operation).