Why did I go from lvl 18 enemies to lvl 26?

I am on the neon arterial part of the campaign, and just finished fighting my way through a whole bunch of COV cultitsts who were all on level then I go down to the vault and encounter a wraith that is lvl 26. Whats up with that?

The main reason this could happen would be higher level players in Co-op other than that . Im not quite sure why this would happen.

Possibly not completing enough side quests .