Sprites & Spell Resistance

[The Spectrum Rider]

All the sprites (although not all the fey) in the MM have SR: grig SR 17, nixie SR 16, pixie SR 15.

Can anyone discern a formula for this, or is it as arbitrary as it looks?

The Spectrum Rider

 

[hong]

SR values are arbitrary, in the sense that there's no defined formula. You pick whatever value seems right for the creature and CR.