Auto Jump will trigger sometimes trigger when it is not needed - particularly when going up stair blocks.  

A secondary effect of this is that this is why going up the stairs takes so much food with autojump enabled. - The autojumping occurs with or without the jump boost, you just don't see the effect aside from hunger most of the time.

To duplicate: somehow get the Jump Boost effect (potion or beacon comes to mind), make some stairs using stair blocks (at leat 5 blocks high). Make sure auto jump is it to on. Walk up (dont' sprint) the stairs. Repeat without auto jump setting enabled. The differences should be obvious.