Yes, but also don't forget that there is an average pax weight calculation as well which may either under or over estimate actual pax weight and so affect actual TOW on the day. Airlines use something like 90kg as an average pax weight.

At the end of the day, the total weight underestimation brought about by heavier carry-on bags is not significant in an absolute weight sense ( 2 tonnes baggage over 400 tonnes aircraft) is 0.5%, but certainly is from a fuel efficiency perspective, and cabin safety perspective.

Having seen 2x JW Black fall from an overhead locker just past my right ear was bad enough, and that is just 2kg!