Sounds like you are using older sensors such as the DHT11 or DHT22. Do yourself a favour and ditch them as they are rubbish. Switch to something like a BME280 or HTU21D.
Just remember that measuring humidity accurately is immensely difficult. Particularly at extremes. The DHT sensors for example not only drift wildly over time, they take a long time to recover from high humidity levels. Relative humidity also depends very heavily on temperature so you get a double issue with a sensor that isn't so accurate. A decent sensor will still show a variance of at least +/- 3% between devices - that is a lot if you are trying to measure differences in that range but will only impact you if trying to compare two sensors.
As an aside, that variance is also a quite lot if you are trying to measure safe levels of internal humidity since the gap considered "safe" generally for living conditions is only around 40% to 60%.
https://www.epa.gov/mold/mold-course-chapter-2
Sorry, as you will realise, this is a bit of a sore subject for me. Living in the North of the UK presents "interesting" issues with humidity, condensation and mold, especially living in an old Victorian house.
