Hmm, sound like a good marketing ploy to me actually. Of course pasture changes, but it's not a reliable change from season to season, not to mention seasons & pastures are different in different areas. Some horses don't get any fresh pasture at certain times & in winter may be eating rich summer hay for eg. While a small amount as a 'ration balancer' or such is likely fine, I also don't like to feed horses such processed rations as a major ingredient of the diet, so would choose something they can chew more, with long fibers, rather than pelleted feed.
There doesn't seem to be an ingredients list, so don't know what's going into it or what levels of nutrients & such. I think balanced nutrition is something important all year, not just adding extra zinc & biotin for eg at certain times of the year. Changing feeds needs to be done gradually, especially if high starch, and as there is no ingredient list, don't know what's in the rest, how different they all are, but it says under 'winter' it is a mix of oats, barley & corn. I wouldn't feed corn to horses anyway, as this is very high in starch & hard to digest. Horses often get less exercise in winter, digestion of forage is the best for warmth, not grain, and they're not built to cope with being mud fat all year - losing some weight in the 'bad' seasons is not a bad thing. Under 'summer' I'm also suss on the 'lower fiber, higher caloric density' too.