The "number of leading 0's" is just a gross oversimplification of the actual difficulty mechanism. It's a lot easier to explain, and it's somewhat correct (it's

The "number of leading 0's" is just a gross oversimplification of the actual difficulty mechanism. It's a lot easier to explain, and it's somewhat correct (it's easy to look at the hash for a block and see that it has a bunch of zeros at the front) The reality is closer to what you say at the end there, it is calculated so that the resulting hash must be under a given value, and is pretty damn granular. Sadly many people trip over the idea of one hash being "less" than another, and it's just easier to go with the "number of zeros" explanation rather than spend time explaining how a hash just represents a bigass number. For example, the current difficulty as of this comment is 1873105475221.611