I had two thermometers (one digital probe and one regular) when I was mashing and they both read the same temp. My mash temp was 152 falling to 150 over the hour. My mash out and sparge temps were low tho. I didn't ever reach more than 160 F.
At those mash temps I'm not surprised at all that you got around 81% attenuation. As said above if you want less in the future bump the mash temp up.