TC Senior and TC Growth are almost identical in profile. I saw no reason to stock two basically interchangeable feeds so the labels would fit the age of the horses better. So when I have a baby on the farm, I just use Senior.
My middle-aged TBs stay on TC Senior because they can eat a lot of it without getting hyper, because of the low NSCs, as others have pointed out.
The horses that only get a little supplement get TC 30%.
OP, my theory is that it is kind of like with human athletes…Michael Phelps, or FEI-fit horses, might need 10,000 calories a day of pasta and pizza, but most of our horses are us really are more like us, and all we really need are need low sugar, moderated diets. We don’t need sports bars and gels to hop around little jumps and trot 20m circles. a banana and a glass of water is probably a lot better choice. Nor do our horses need the same diets as their upper-level companions unless they really are in that kind of work.