On servers with auto-difficulty, the difficulty is automatically adjusted by the server. If all players ON AVERAGE play like noobs, they won't get a difficulty higher than EASY (10%). If, on the contrary, a bunch of "PRO" players have gathered, they are guaranteed HARD (50%) or EXPERT (100%) difficulty. But as practice shows - 95% of players in this game's community CANNOT HANDLE EXPERT. As soon as the surprise "EXPERT" appears, almost everyone crumbles within a couple of minutes.

ATTENTION, EXPERT on BadBlood - is not the one everyone is used to, where bosses eat you in 10 seconds. On expert, you'll drop in a pile of dust in a couple of seconds. The Hunter will devour you in a couple of seconds. The Charger can smash you against a wall to death. The Tank will practically kill you with a rock. Well, you get the idea.

As soon as a critical number of players who can play on EXPERT gathers - a server with only EXPERT difficulty will appear.

More details about the difficulty system are written here: https://steamcommunity.com/groups/l4d2badblood/discussions/3/737036097948946727/