No doubt the local Forge World ran out of full-sized Skitarii-brand prosthetics. That batch got dealt child-grade shit. Then for a giggle they got sent to deal with a Custodes, who're taller than Astartes.
So you've got some dudes with undersized legs from the knees down and some dudes who're, like, 10' on average (IIRC Astartes are like 7 without gear, and there was that one dude from the... Iron Snakes, I think?... who was like 10 and needed specially crafted gear because his CHADliness was just overpowering)