Dungeon Cards

[Paull51878]

Hello

Im a little confused in regards to dungeon cards. The rules say The Dungeon cards form their own deck. Start the
game with one Dungeon card turned face-up. This is the
Dungeon that you are in. To choose it, you can either:
(1) Pick one randomly, or
(2) Let the player who won the last game pick a
starting Dungeon, or
(3) Let the player who LOST the last game pick a
starting Dungeon, or
(4) Just agree on one you like.
However you pick the first Dungeon card, it’s likely
to change. So shuffle the rest of the Dungeons and
put the deck face-down, ready for the moment the
munchkins stumble through a Portal . . . NEW CARD TYPE – PORTAL

My confusion is that it says start with one dungeon card turned face up. This is the dungeon you are in.To choose it you can pick randomly, etc. But if this is the dungoen you are in, why wouyld you pick one randomly its already face up in use?

Maybe im missing something, please help

thanks

Paul