I don't know this for sure, but it might somewhat explain your turtle's shell.
If you look, the older alligator snappers get, the more smoothed out their shells get as well. The enormous century-old ones only have three raised lines down the back of their carapace rather than lots of individual spikes. I think their shells can just vary in texture.
Another thing that varies a lot among this species is coloration. For example, Crunch, the world's largest captive alligator snapping turtle, is really light colored (almost entirely white). However, other specimens are grey, black, brown, reddish brown, green, etc. I think your AST might just be changing naturally.