Case hardening isn't an applied finish, it's like a heat quench. Like a box of chocolate, you never know for sure what your gonna get.
That being said, a closer look seems to be in order so when spots are completely missed it gets a do over or something.

For a long time you never saw a case hardened slide, only frames. I always figured the heat would mess with the hardness or something on the slide. They must have figured out how to do it without negative effects.