My feeling is........ with cams that have extremely slow closing ramps, some compression starts to build before the valve is actually “seated”.
The valve is closing, the piston is ascending....... and if the piston is displacing more area than can squeeze through the small opening the almost seated valve has provided, some pressure will start to build.
So, if you tested two cams with the same exact actual closing points, but had radically different closing ramps(one really slow vs one that just slams the valve down), they would show different readings on the compression tester.
Although the calculation would say the cranking pressure would be the same.
It’s the same reason that a 292* cam could have 20* of vacuum.
The number of overlap degrees might be large, but the area under the overlap curve is still rather small because the valves aren’t open very far.
This 292/292-112 cam has the same overlap as the MP 484 cam, which is 284/284-108. Both have 68*.
Think that 484 cam would idle at 20” in this 318?