True, we can't blame MSC for the pax sailing and their behavior- that can vary for week to week.
We sailed a B2B on Celebrity and the one Officer told us to expect a big difference with the 6 day vs the 6 day--and yes he was very correct. Difference in tone, demographics etc.
So length of sailing, time of year, port of embarkation all impact who may be sailing.
Booze cruise vs longer sailing. Spring break, holidays, summer vs "in school", port intensive vs lots of sea days, does the ship have lots of "attractions" (surf-riders/race cars/arcade), is it a big casino draw etc