The most likely cause is you're using your gauge improperly.
You said you measured anywhere from 8lbs to 10lbs. A 2lb swing is extremely unlikely. The M&P isn't the most consistent on the market, but that's almost impossible.
You are not the first to have this problem. It has to do with the way a trigger pull gauge is used and the weird hinged trigger on the M&P. If you try to pull the trigger gauge straight back, parallel with the slide, it doesn't move the lower part of the trigger enough. Thus the trigger safety tang gets partially caught on the frame.
Try measuring it again. This time, take out all the slack first before you start pulling on the gauge. I'll bet you get a most consistent measurement.