Unexpected State Refund

I used TT to file my taxes, as usual.  I significantly under-withheld, and had a balance due of approx. $6,000 with my return.  Turbo Tax calculated that I owed a penalty of $291, which was included in the amount due.  

 

About 2 weeks after filing the return, I received a check from the State for $291, without any explanation - just a check.  

 

Since the numbers match, I'd say this was obviously a refund of the penalty.  But why did TT calculate that I owed a penalty when the State effectively rejected it?