The 747 had 7 tanks too.  ;)

The answer could be that those planes had a flight engineer and of course you need to give the F/E something to do.  ;D

A few of the early 767's we had at TWA had a F/E position. That was soon eliminated and the F/E turned into the "eater." That is what the second pilot's position was called even on the bid sheet since all they did was eat!  :o  The real position's name was relief pilot.