For starters, one doesn't always have a weighing scale wherever one is. I think people can tell the difference between a 30kg bag and a 7kg bag, but I wouldn't know whether my bag weighs 5, 7 or 10 kgs. It's a guideline. Even if I attempt to pack 7kgs I might end up packing 9kgs. Draw the line by being a good judge of whether someone is trying to abuse the system, or whether they just have a couple of extra things.

For example, airlines with a 20kg baggage allowance will usually let you check in up to 23kgs. It's an unpublished 'line' they draw, which makes customers carrying between 20 and 23 kgs feel like the airline is being reasonable.

Airline contract of carriage absolves them of any responsibility, apart from getting you from A to B. If crew can 'break' a rule by sending me a glass of champagne in Y, it isn't unreasonable for check in staff to overlook the odd extra kilo.