Unless you can't lift the carry on bag into the bin, how would they know what it weighs? The only time I saw carry on being weighed was on the jetway boarding a SQ flight, where a guy in business class was getting a hard time about an overstuffed roller bag. QF has a very low limit and they once commented to me near security on how they have this rule unlike in the USA, but my bag got onto the plane despite being over five kilos or whatever the limit was.