Unfair costs on baggage, not understanding client

The most unfair way to overcharge a client and try extract more money out of them. My wife and I travelled to the UK and on returning we had one bag over the allowed weight limit per bag. In all due fairness it does warn you about the limits and the charge. Here however is the lack of BA in understanding their customers. Myself and my wife had 2 bags, one large one and one smaller one. Combined, these two bags were under the limit allowed for both of us. Ie. we were allowed 23kgs per bag. The two bags came to 45kgs which is obviously under the 46kg allowance for both, however one bag was 27kgs and the other 18kgs. You can see that one bag was 4kg over the allowed limit per bag. We got hit with a charge of 65pounds or R1 300 to get this bag back to SA. This is ludicrous. Firstly not even mentioning that together we were under the limit as a couple travelling but secondly how do not understand that as a couple we take one big bag and one smaller bag, and you cannot shift the weight because the bags are different sizes. Furthermore have you ever seen a set of bags being sold in a luggage store that are the same size, NO, they sold as one big one and one smaller one.