Ok, here's my 2 cents worth, have the engine warm if possible, remove the spark plugs, open the throttle wide. The function of the throttle plate is to restrict the air flow into the cylinders to maintain the proper air/fuel ratio for combustion. If the throttle is in idle position then air flow is being restricted and the resulting readings could be lower. The proper way to do the compression test is with the throttle wide open so that airflow is unrestricted into the cylinder.
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