Humidity and hygrometer issues . . .

I have two hygrometers in my incubator and I have dicovered an oddity. Since they are both the cheap looking digital type I made sure I checked their calibration and I was amazed to find that they were both only off by 5% (both read 70% and i put them in bags with the same cup of salt water mixture). So thinking all was pretty accurate I put them both in the incubator and all of a sudden one is reading 22% and the other is at 54%???
Can someone please explain how/why this is happening? It is a hovabator 1602N with a fan, the temps have been lovely and sit between 99.5 and 100.5 and I'm on day 4 with 41 eggs in the turner. I haven't put any water in as our normal humidity sits around 50%. The only difference I can see between the units is that one says Relative Humidity and has a silver looking probe, and the other just says humidity and has a little bulb looking probe - thats the one that came with the bator.
Any ideas or suggestions?