I suspect the fees aren't always about cost, but rather a subtle encouragement to customers to do certain activities. I had a bike bag weighing 27kg and a seco

I suspect the fees aren't always about cost, but rather a subtle encouragement to customers to do certain activities. I had a bike bag weighing 27kg and a second piece weighing 15kg. Well under the 46kg I could take on for no charge. The airline (Qantas) wanted to charge me an overweight charge for the bike bag - but were happy for me to park in front of the counter for 5 minutes while I moved heavy items between bags to balance them out. I didn't really care; it was a financial motivation to keep my weights at an easier to manage level. I imagine that overweight bags require different workers, who might be on a different pay scale - and so there is a cost to moving 27+15kg vs 22kg+20kg.