honey slows down when running over it and has other delaying abilities but if items are swept over honey blocks by water currents, the items only slow down in the last two blocks of the current (7 and 8). that's inconsistent. It should have that delay all the way through.
This is the same for all blocks and it is because items float in water. They slow down on the 7th and 8th blocks because the water is too shallow there to keep them from touching the block. See MCPE-36961.