In Bulk (or previously converted), Calibre will default to using the last
used setting for that book.
There is a tick box to clear those previously used (greyed out when none of the books have a saved setting)
If no setting is in force, Preferences takes over.
As you guessed, Same format is not a normal choice.
follow itimis advise. Have all your ducks in a row
before doing your first conversion pass.