Unfortunately Tim most of the thermometers out there are not actually that accurate especially the digital type. They are normally only accurate to + or - 1c at best so potentially up to 2c out. I Have a Rueben Heaton and found it to be the worst but at least is consistent in it's inaccuracy if you get my drift. The first one I had read 4c high, same with the second one, the third one around 1.6c high. I just don't rely on it.
If you collate your readings with your mates you should at least see if your readings are consistently out to his readings, assuming of course you are both taking temps on the same bit of river at the same time, and who is to say that his is correct anyway.
Whatever the temperature the thing that usually matters most is whether the temp is rising, falling or stable.