Covering polluted water with more polluted water can slow down offgassing

Expected:

Increasing amount of polluted water in a container increases the amount of offgassing polluted oxygen, except in the cases where it makes it reach the ceiling.

Actual:

Only the amount in topmost layer contributes to offgassing. Covering a 800kg/tile lake with 1kg/tile layer (also of polluted water) will slow down offgassing to the rate of 1kg/tile layer on top.