Originally Posted by
croberts134
There are only 3 seats available at the lower fare. When you add the 4th passenger all passengers are charged the higher fare since there cannot be multiple fares on a single reservation. The best bet when that happens is to book 3 on one reservation and then 1 on the other.
It's almost certainly this.
The difference between a discounted "F" fare and a full "P" fare can be quite large, and of course when that difference gets multipled across 4 pax, we're talking $$$$$$$$.
AF.com (and many other site) will have a note next to a specific flight saying something like "Only X seats left at this fare". I think for AF that cutoff is 4 seats for J and 3 seats for F, although perhaps it depends on the configuration. It does seem that for many 77W flights AF will initially offer P4F3, so you wouldn't be able to get the whole family the cheaper F fare - you either have to look for a 380 with a larger P cabin or just suck up the higher P fare for that last passenger.