I really think the people at check-in just make it up on the spot. Its annoying enough when they are really strict (yes, I understand its for my own safety, but I know its at their discretion, because plenty of times they've let me get away with being a bit over. Singapore was the worst/best - they were going to charge me £800 for my excess baggage until a rep from the arabic airline was called over. He shrugged his shoulders and wondered off, giving implicit permission to bend the rules to the tune of £800. So either the plane was very dangerously overweight, or it doesn't actually matter that much) but the worst is when it changes on your flight home! "The allowance is 20kg? But on the way here it was 30kg! And I've eaten all the snacks and used all my shower gel, so this luggage is even lighter than on the way out!". Whatever.
