...they simply decided on the spot that they would show it in 4 screens instead of 3 to accommodate our massive group.
That makes sense if they have the option.
During the first days of a film's opening, the studio takes the biggest percentage of ticket sales - that declines over the weeks, so a movie with "legs" is more profitable for the exhibitors. Given that, the problem with selling out the house, particularly weeks in advance, is that for any film you'll have a sizable percentage of no-shows (life happens while you're making other plans, y'know). And no-shows don't buy expensive candy and popcorn at the concession stand.