I actually didn't notice this till someone pointed it out. If I had to guess I would say their alien body chemistry means acid doesn't react with them or something like that.
They often come from strange planets or dimensions with environments which seem like acid to Earthly or Golarionly beings. Acid immunity covers that.
What's it gonna do to 'em? Make their body more heinously twisted?
I mean, if you told me shoggoths had acid blood, I wouldn't say "that doesn't sound right".
Look at a picture of one. Does it look remotely basic to you?
*slow clap*
I believe that in the rules "acid damage" represent effects of both chemical acids and chemical bases (like quicklime). That's why earth element is associated with acid damage - although few rocks are actually acidic, some of them are strong bases. In short "acid damage" = "chemical damage".
Having that in mind, acid resitance is an ability to be unaffected by all kinds of chemical burns. In that regard it fits for aberration. If your body is flesh and blood, there's little that you can do, but aberrations can have a different body composition that is more resistant.