Well, I'm going off of a d20.

If you roll a 1, it's dangerous as hell, you'll see high levels, but not just any. They're aggressive.

That said, you can ask for a different 'favorable' situation if a 20 is rolled, like rather than loads of commons, or lowered rarity, you can try and ask for a specific encounter.

It's just a little fun mechanic for adventuring.