Agents could follow it but my experience is generally they do not for business. If you are looking at taking three bags or two overweight bags then you are going to draw sufficient attention to yourself to be more likely than not to being charged. Likewise I find the ground staff in Adelaide to be especially vigilant.

@Dave Noble I am not sure the system actually knows, just had a friend fly on the Air France code and QF tried charging him the QF excess bag rates, not the AF rates as per his ticket. Wasn't until the agent when into the ticket that the correct, much lower rate was charged.