Its a few factors...
Genetics do play a role with basics of shape, length and fullness.
Water quality (temp, pH, ammonia, nitrite, nitrate), injury and cannibalism are huge.
Fungus from stress and water quality will damage gills and they may or may not grow back.
Nips from fish, other axolotls etc can cause uneven/short gills
When I grow out babies you see how all the babies tend to have similar length and fluffiness gills in a clutch but the next clutch has their own gill traits.
they tend to loss fluff and length from water quality and if kept with other juvies that nip - it takes a while before they stop being so nippy.