This has been a bug for quite some time.
On 1.8 pvp servers I noticed me and other players being unable to hit each other while we were both falling now i know why.

While a player/ any entity is falling their hitbox is invisible and located a few blocks below the entity. The said hitbox cannot be interacted with by a player and the only way to hit an entiry is fire a projectile a few blocks below it to hit the invisible hitbox. I assume this is caused by dsynch.

I also assume this bug is present in all multiplayer versions of minecraft and in all singleplayer worlds after 1.2

Effect of this bug:
-You are unable to hit another player/mob while you are both falling
-Other players and mobs also cannot attack you
-Arrows phase trough the mobs while it looks like it should hit them
-You cannot interact with any mobs that can normally be interacted with
-Shooting an arrow few blocks below the entity will cause a hit

I have a video explaining everything below.
Explanation video