Why the difference in PSI turning by hand or by starter?
Simple, physics...
When you turn by hand, you can only turn the engine so fast. (as fast as you can pull)
When the starter turns over the engine, it turns it over at a higher rate and constant speed.
How does this effect the compression test?
Ports...
When you're turning over by hand, the compressed air from the bottom of the crankcase is released into the cylinder, and because you can only pull it over so fast, most escapes out the exhaust. And since there's not nearly enough speed/pressure it does not get crammed back by the pipe, like a running engine would.
When you're cranking by starter, not as much air escapes out the exhaust port because the piston closes off the port faster while the cylinder still contains more of the compressed air from the bottom.
This is why when you test a piston-port engine with a broken piston skirt, you will not see a difference in compression. You're turning over slowly enough that the compressed air is escaping, and all that's captured for the test is straight piston compression.