I'm with Jerry on this, usually air/steam pockets make it read on the high side. The thread sealant increases the resistance, the sending unit is a heat sensitive resistor the more resistance the cooler the gauge reads. Of course there's always the possibility that the gauge is out of calibration or it has a mismatched sending unit.
And as mentioned previously, real easy to check the actual temperature with a laser, most shops will read it with their laser for free (or very low fee).