Gaps in collision between vertically stacked cacti

Cacti have a collision box of (1/16, 0, 1/16) to (15/16, 15/16, 15/16), which allows players to take damage on cacti they stand on. However, because the collision does not take up a full block vertically, the 1/16 empty collision at the top of each cacti block will repeat for each block in a cactus, even if they aren't the top block.

Since cacti visually appear as a solid box extending vertically, I wouldn't expect there to be gaps in the collision associated with this box. However, there indeed are gaps, and e.g. arrows can be shot through these gaps. I would expect the reduced Y height of the cactus' collision box to only exist for the top cacti.