It is a random generator the Pokemon can have a 1-5% to appear for example purrloin can have a 3% chance to appear while boldore can have a 1% chance to appear. It is random for every game though so darmanitan could have a 5% chance in your game but 1% in someone else's game.
Source: My friend's uncle took us to gamefreak studios(nintendo america) and I got the privilege to ask a person that works on wild Pokemon this question.
(AND IT WAS AWESOME :D)