If you are doing this the way I think you are, the numbers will be low due to the closed throttle and high vacuum.... I would expect very incomplete cylinder filling results at actual running speeds versus cranking speeds, plus not-so-great cylinder clearing due to the mixture not being burned and the internal pressures being low on the exhaust stroke. So the absolute numbers in this test are pretty much meaningless compared to what we normally use (cranking pressures, taken at a very low engine RPM); the comparative numbers between cylinders are what the OP is after in this test.
I suspect you are correct on the lifters. It's nice that a real measurement verified your work to get the cylinders to even compression.
