It's more accurate to say "picking one of the right numbers" which has to be below a certain threshold. Difficulty is how low that threshold is. Imagine a hat with a million numbers in it. Its much easier to find one < 100,000 than finding one <10

Difficulty is the threshold of correct number. If there is a realized reachrate of 1 per 10 minutes then the difficulty is 100% - any number is a correct number. If the realized reachrate is calculated as 10 per 10 minutes then the difficulty is the top 10% of numbers in the hat... and so on.

mining difficulty

natural selection