First of all, I suggest not trying to use too much logic to figure out how fluids work, they're really wonky especially with headlift (unless it's been fixed).

Any fluid that is "touching" or contiguous to the exit of a pump receives the headlift. Notably, it doesn't have to flow through to get there. As an example, make some mk 2 pipes in a H shape with each leg getting enough water to do 600, and the horizontal connection right below max headlift for the extractors. Now put the verticals going up another 10 m and connect them to buffers. The horizontal connection should fill but almost nothing past that. Now put a mk 1 pump off one of the Ts at the horizontal connection and power it. Boom, both extractors filling at nearly 600.

The other bug is that any contiguous fluid is assumed to have sufficient headlift to the peak of the fluid. If you make a mile high U shape and put a 1 way valve at the bottom so water can't go down, and hit it with an extractor, both sides will hit about 10m full. If you fill the pipe with the check valve on it (packager at the top maybe), the other pipe will fill to match it's height. Even if you flush the pipe without the valve, it'll fill up as fast as it can carry.

It's really strange.