My count is 24 unused letters for the grid total, 0pt0 - so, one of us has miscounted (but it could be me because I'm prone to error in that department).
I assume you're then counting the number of different letters (alphabetically) featured in the block of 9, and counting only 4. This must be wrong, because there are 6 different letters in the block of 9 - one letter is doubled and another is tripled (the two multiples making 5, plus 4 singles, 9 in all).
Or have I misunderstood your question altogether? (I'm sorry, if that's the case, but I'm heading out now - hopefully, buddy will get back to you in the meantime.)