Having enchanted and disenchanted around a hundred of each piece of armour (all materials as well!) and even books, I discovered there wasn't a single occasion where Projectile Protection (of any level) came up in the 3rd/bottom enchant slot with an enchanting setup capable of giving Level 30 enchants.

On very, very rare occasions, Projectile Protection IV came up on the 2nd/middle enchant slot but never once on the bottom slot. The rest of the protection related enchants (Protection IV, Fire Protection IV and Blast Protection IV) show up as normal, just never Projectile Protection.