Why is spell resistance all over the map in the Bestiaries?

[Nightwish]

I've always heard that standard spell resistance for creatures that have it is 11 plus hit dice. But thumbing through both bestiaries, I'm finding everything from 8 plus hit dice to 12 plus hit dice. Is there a rhyme or reason to why those are so all over the map, or are they assigned arbitrarily?

[Are]

The most common SR is 11 plus CR, rather than 11 plus HD (all Demons, Devils, and Dragons have this common SR, for instance).

Some monsters have different SR, based on if the designers felt the monster should have a little more weakness to magic, or be a little stronger against magic.