What kind of tester. Screwin or hold it on the hole? This is the best way I've found:

Screw in tester with reading hold button
Throttle blocked all the way open.
Battery full fat charged up with charger on it while you test the compression?
All spark plugs out and ground the coil wire.
Test for min. 300rpms cranking speed
Test each cylinder in turn, recording readings, pausing to let the batt. recharge after every cylinder. I like to get 4-5 compression pulses per reading, but I think you're supposed to crank it until the reading peaks, but less than 10 compression strokes ("chuff" sounds while cranking).

I know of almost nothing that would cause an engine to run fairly ok, then have that kind of compression on every cylinder except a massively and multiply cracked block, or a non spinning cam cocked with all the valves open just a tad and/or all the valves bent by a similar unlikely amount. I am therefore guessing measuring error, no offense intended....