My luck or not?

I was hunting for a Drilbur in Midna Mine and I caught 4 of them, which is good but the only problem is that 2 of them have a Calm nature and the other 2 have a Bold nature. Is this just my luck or do those Drilburs always have that nature?

EDIT: just caught a Brave nature one. It was just my luck

Just your luck. Try breeding them or Synchronizing the nature (have a Pokemon with Synchronize in the front of the party with the nature u want the Drilbur to have).