Nowadays, with many “normal” style hyd lifters being used with a HP cam and the associated spring loads that go with it ...... when you’re adjusting lifter preload with the motor hot ...... it’s not at all uncommon to bump the motor over to put a valve at full lift ....... and if you just keep an eye on that valve, you can watch the spring load start to collapse the lifter.

Anything less than fully pumped up will affect the number(make the number higher) you see on the compression tester.

Depending on exactly how much the lifters bleed off during the cycling of the valves opening and closing during the test can give you erroneous info, in terms of knowing what the “running” compression would be when the lifters are operating with full oil pressure.

Be an interesting test, especially if one were using some of the variable duration style(fast bleed) lifters.

Test a few cylinders after the motor has sat for a while........ then pull the oil pump drive gear, and use a priming tool while retesting those same cylinders........ see how the numbers compare.