Indoor humidity gauges vs outdoor in iss unit on roof

Hello,

Ive been calibrating my aercus weathersleuth, similar to watson 8681 pro, the outdoor iss is on my roof on a pole and about 30 feet off the ground. Ive had to offset the reading a bit but it appears to be reasonably accurate.

I have 3 indoor humidity gauges and I put them all outside today on a table, all 3 read aprox 50% RH, whilst the iss was reading 65% RH. I then took 1 of the indoor sensors up on the roof and left it there for over an hour. It still read 50% RH, right next to the iss reading 65% RH.

So my question is, are outdoor and indoor humidity gauges calibrated completely differently ?

Michael

Edit: thought I would update this, I have since learned that humidity can vary by quite a lot, even in the same location.