CX' web site mentions a 7kg carryon weight limit for economy class passengers.

Any particular reason for this, since it does not seem to be strictly enforced? I brought a carryon that was within size limits, but was 8kg (though packed with a small bag inside to carry on if they really did force me to check it). At one check in, they weighed it, but still let me carry it on. At two other check ins, they did not ask to weigh it.