This might be a long shot, but to my knowledge, LTU operates both a 3-class configured 330 to MIA (D-AERD) and regular ones with economy class (or 2 classes: LTU First Comfort and Coach?) seating. "Just" might therefore mean that an aircraft without 3-class seating is available for this particular flight only.
I'm not Sherlock Holmes,
but I hope this helps,
Homer: Facts are meaningless. You could use facts to prove anything that's even remotely true!