A couple of points here AAaLot

First, in some locations (New Zealand being one) the agents apply the bagguage size/weight rules more strictly.

Second, SQ is not keen on large carry on bags. They are considered a danger to other Pax. As such, in some locations, like Singapore, the guards on the flight ramp if they suspect your bag is too big will remove it from you before you board the plane and throw it into the planes belly.

The moral of the story is avoid large carry-ons. A better plan is a larger check in and a smaller carry on. As far as check in bagguage weight is concerned SQ will give you 10% and often 20% more than the stated limit.