when I was working in the Pharmaceutical business we had a Benchmark thermometer, when a new thermometer came in it was compared with the Benchmark and the difference was then marked. Once you know the deviation it doesn't matter as you can compensate. The deviation should always be the same though, if that's not the case it's time for a new thermometer. Like people said here before, immersion depth is important as is looking at the scale in direct line and not at an angled line.

It's not uncommon to find deviations, even in new thermometers. You can buy them with guaranteed 0.1 deviation at most but they will cost you dearly, those are needed for very precise analytical procedures. For photographic development a 1C deviation isn't a disaster as long as you know it's there.