Mixed Sets?

[Tradedollarnut]

I wonder why NGC has some Registry sets set up with mixed proofs and circulation strikes? Is there some criteria used to determine which sets are eligible?

 

IMO, it'd be neat to have that option on every set! But I do think that the mixed sets should be separate from the proof or circulation strike sets. Notice that in the flying eagle cents, the proof sets overwhelm the circ strike sets - is that really right?:

 

Mixed Proof and MS Flying Eagle Cents