Other coins miners including Bitcoin and Ethereum calculate hash of some data that should be within some limitations. Miner for Zcash uses EquiHash algorithm, where not only lot of hashes calculated, but they also should be sorted and compared to find similar pairs (thus the algorithm name EquiHash). Such found pair is a solution to some math problem, more than just a single hash. So actually miner sends to pool such solutions. But they often called hashes while they are in reality more than just hashes.

This is also the reason why old miners used a lot of CPU: GPU calculated a lot of hashes and passed them all to CPU to sort and compare. Latest miner versions offload CPU and do this on the GPU directly returning just good solutions. But this also explains why there is no Dual miner for this: because now both GPU is busy calculating hashes and VRAM is also busy for sorting huge lists of them.