5-7 psi at warm idle is OK, low but OK to prevent bearing/engine damage.
It is odd on a rebuild with new bearings through.
Oil pressure is the back pressure as oil is forced out the bearing gaps, so not pump pressure.
As long as there is any pressure oil is lubing and cooling the bearings.
Valve train noise at idle would be when a real concern would be warranted, that would happen at about 2-3psi
As to why the lower pressure, oil distribution is like a soaker hose with alot of small holes in it.
As long as the Water pressure in the hose is above the amount of water that can pass out through the small holes water flows at each hole, thats back pressure.
If you were to make only one hole larger, then the back pressure at all the holes will go down.
So if the rebuilders failed to install the correct size bearing for the journal wear on one bearing then back pressure in the whole system drops.
So you have a "larger hole", but just guessing.
You can run a "thicker" oil if the pressure is a concern, 10-40 would be fine.