Humidistat readings vary a lot. Here is a photo of three humidistats that were all taken in the same room. The wall controller humidistat is part of a good quality 10+ year old room humidifier. The Meade humidistat is a good quality home humidistat, and the Fluke humidistat is an expensive industrial grade humidistat that is very accurate. As you can see from the photo they range from around 34%-49% relative humidity.
The $10 humidistats you get for your guitar case are good to have, but periodically check them against a high quality humidistat. The system we use in our Guitar Habitat™ is accurate to within 2% on the digital controller, or 5% on the analog controller. Either is accurate enough for maintaining proper humidity levels.