Conduit attack speed is inconsistent with distance

The damage from a conduit does not damage at the same rate at the range of 8 blocks.

Steps:

Create functional Conduit

Place Zombies in water at both 7 blocks (or anywhere between that point and the conduit) and 8 blocks away.

Observation: It takes 10 seconds (as expected, with 4 health or two hearts every two seconds) for Zombie to die at 7 blocks, but about twice as long for the zombie at 8 blocks to die (with longer periods of time between damage ticks).

Expectation:  For mobs to take damage at the same rate, regardless of distance.