Slightly different scenario but on the same line... I was recently charged excess baggage on my 2 pieces allowance for DONE4.

The weight of the two came in at 54kg on CX's scales. The check-in agent insisted I put both bags on the scales at once.

My ticket says 30kg, but the way I understood it was that because my itinerary included travel to North America I was allowed 2 pieces of 30kg each.

Had no trouble with the same 2 pieces, perhaps slightly under that weight on every other flight I've taken so far with QF, AA, BA and one earlier one on CX (checked thru by BA however).