You are of course quite right looking at it from a "black and white" perspective but given that the size and weight limits are probably imposed for two reasons 1. to ensure the bag can fit in the overhead and without it being so heavy that the overhead locker does not break off and 2. that airlines want to manage the amount of weight that is carried (including, people, checked in bags, carry on baggage, freight and fuel) then perhaps a little leeway at check in could be granted when a person has not already checked in their limit (if indeed they have checked in anything at all) and the carry on bag is an acceptable size and shape and especially if the additional "personal" item(s) is not being carried. The fact that there are also differences between carriers in what is an acceptable weight for a carry on bag also might be something to take into account. I remember when BA were impossibly difficult about the weight of carry ons but now they (to the best of my knowledge) allow you to carry on an item of any weight as long as it does not exceed their dimensions rule and you are capable of stowing it in the overhead yourself. Anyway just a few random thoughts on the subject without getting into suggesting that people of slight build might feel entitled to a bit more leeway that those "of size" as they say in the USA - presupposing that the main aim overall is to manage the take off weight of the aircraft. I really think that, unlike some people like to suggest, "size matters" and that is what should be adhered to to ensure that bags fit comfortably into the overheads. However, what I think does not mean others will agree, especially the powers that be in the airline industry, but that is another very long and sad story.