Not quite what it seems.
You have to subtract the officers who are (well) salaried.
Also, subtract engine room crew and others who are not part of the 'tip share out'.
That could cut in half the number of crew included in the equation.
Seeing as they already suggest $15 a day - then it would probably be an extra $5 a day.
Considering what I spend per week on tipping for drinks etc where a 15% gratuity is added - I'e be MUCH better off (and they worse) under the 'included in the price' idea.
Any wonder they don't include it?