I'm curious how often productions release mezzanine and balcony seats to TKTS. Admittedly I don't use them all that often, but off the top of my head the last TKTS seat that I had that wasn't in the orchestra was for Chicago, and that was back when it was still at the Shubert! (Like I said, I don't use TKTS very often, so my personal sample pool is pretty small)
Thinking it out, and this is just my own logic, it would make sense for the box office to hold on to the cheaper seats. That way they can sort of "down sell" when they have somebody at the box office getting those last minute tickets who might balk over a full price ticket. "Oh, that's kind of expensive, do you have anything cheaper?" "Well, we do have this seat in the mezzanine/balcony."