In practice Bitcoin treats SHA-256 outputs as integers, and requires them to be under a certain limit, based on the current difficulty. That way we can make sm

In practice Bitcoin treats SHA-256 outputs as integers, and requires them to be under a certain limit, based on the current difficulty. That way we can make small adjustments to the difficulty. If we actually used a rule like "this many zeros in hex", the difficulty bumps would have to be rather large.