Sensors drift over time. When I replaced my temp/humidity sensor both my temp and dew dropped noticeably when compared to the Davis's surrounding me.
True. I haven't looked at the drift specs, but the SHT2x family specs have improved considerably over the last year or so, too. The SHT21 used to be +/- 5 RH percentage points, now it's down under 3. That's assuming identical reporting temperatures, of course, as the two numbers are inextricably linked.