Yes, the EGT probe is calibrated for a certain temp, lets say 70 degrees, and gauge indications are inversely proportional from that point.
Let's say that your underhood temp is 100, 30 degrees higher than the calibrated temp, your EGT gauge will show thirty degrees lower than actual.
In reality, while driving down the road, it isn't a make-or-break difference, but it's something to consider if you're pushing the limits real hard.
Randy