I would say that "Maximun Load Capacity" would be the total weight of the tripod head, camera body and lens.. I would say that 26 lbs. would easily hold a ball head, Camera body and a 500 f.4 or 600 f4 lens with no problem..JMHO
It could have something to do with how the weight is distributed. It may be the longer lenses need a tripod with a larger foot print to distribute the weight more evenly. A larger lens on a smaller tripod may have stability issues regardless of MLC.
Note that we add a fair load to the rig when using it and stabilising it with our bodies (e.g. arm on the lens, hand and face pushing on the camera).
Also note that the MLC is usually in terms of weight centred on the tripod (same goes for the load ratings of heads). In fact the GT2542LS actually has a "safety payload" of 25 kg (55 lb). With a gimbal head we try to centre the weight of the rig over the tripod, but even so with a long lens there can be a lot of rotational force as well as instantaneous off-centre forces. It's not just a matter of how wide the feet can go.
I'm sure these manufacturers have found it safer to be cautious with their estimates of what you should put on the tripod. But at the same time I've suspended 110 kg from the centre of a GT3541 for short periods with no problems, which is waaay over the numbers.