State (CA) just refunded some of my tax paid - why?

Turbotax claims 100% accuracy, but I after filing and paying taxes due to both federal and state a few weeks ago, I just received a tax refund from the State of California. No explanation - just a check. (a part of what I just paid).

 

It looks like Turbotax got something wrong - luckily, in my favor - but how do I find out what happened?

 

If I'd owed money instead, I guess I could have used my guarantee, but even in this case, with no explanation,  I have concerns about TTax accuracy going forward.

 

(The product was Turbotax Home and Business, by the way)

 

Thanks!