It's RNG.
I've been running HoH with another person from my FC (not for the hairstyle, incidentally). We're up to something like 50 runs and ~100 bags each.
In the first ten bags he got the hairstyle twice while all I got is Materia and fireworks, after that we didn't see the hairstyle again, just minions for him and chocobo bardings for me.
RNG is weird sometimes.