It's pressure.
The oil comming from the oil galley plug (which is roughly 3/8" hole) will be less.

the hole for the pressure unit is about 1/8".

Just like a garden hose, put your finger over the end and the pressure goes up.

Plug everything up, get a pressure guage, screw it in the hole for the pressure unit, use a priming tool, or an old dizzy, and a drill and spin it!
You may need to rotate the crank a bit to make sure everyhole is open.