Poor compression in one cylander at idle.

Stange situation. My daughters '63 Valiant has fairly consistant compression (125-130) on all but 1 cylinder, which is down at about 90. At idle the cylinder is missing, but it kicks in around 2000 rpm. Carbon build-up, sticky rings or worn cam were suspected.
A local mechanic (who knows the older cars) performed a leak test that held fine. So we assume no build-up problem.
Then he added oil directly into the cylinder and the compression jumped to almost 140. That should suggest no worn cam?
He did some standard cleaning treatments that did nothing. Plugs, wires, dist.cap have been changed and re-checked. The engine sounds great, with no knocking.
My daughter has been driving with an oil addative, but so far no change. So now we are scratching our heads.
The mechanic thought, maybe the compression ring is froze but the secondary ring is good. But he agrees that isn't a great explanation.
Thank you in advance for any thoughts or input you might have.
Rjg