When firing effect arrows from a stack in your inventory (or the offhand slot) the game seems to think you have fired the whole stack, as when you pick up the arrow it gives you back the amount in the original stack.
For example:
Fire a regen arrow out of a stack of 32.
You now have 31 arrows.
Pick up the arrow from the ground.
You now have 63 arrows.
This is not the case with normal or spectral arrows.