Are you certain your compression gauge is accurate? I have 4 compression gauges; 3 read very low compared to my good SnapOn gauge. I have compared a few friends gauges, too. Almost all of them read low, sometimes by as much as 30psi.
Sometimes it's down to where the schrader valve in the gauge is. It should be at the end of the hose, where it screws into the head. Cheaper gauges tend to put the valve at the gauge itself; this adds the volume of the hose to the head cc's, throwing off the test.
More than one gauge I discovered had a bad schrader valve, which will also cause very low readings.