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Try lubing and using a small screwdriver shaft to stretch your O-ring on the piston. I've found odd extreme variations such as your's with ported piston heads, namely new ones. Stick the small scredriver under the O-ring, get the shaft under it and pull the ring about 1/2" away from the head, then rotate the screwdriver around the head a half dozen times to make the O-ring stretch out. Could just be that sometimes there is a bit of compression, enough to inflate the O-ring to give higher readings, and other times not enough causing lower readings.
Could also be that the piston is sticking (as above), found that before too, and could be improper sealing between the air nozzle and the hop up rubber.
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