The hashes not being there isn't particularly relevant within a hash function outputting a specific size. If your hash function is always 64 bits for example, the fact that you have 3/4th of them not exist means you should be operating as if its a 16 bit hash, not a 64 bit hash. If you still do this math based on the 64 bits outputted (2^64 boxes) you'd arrive at very inaccurate numbers.