People forget that not all climates exist equally too. I live in Idaho and it is very dry here so I had a pretty sizable cup of water in my incubator, but it still took a few weeks to get the amount of surface area just right. My Hydrometer was reading about 50-55% when I got it right, I know the humidity was correct because of the weight loss in the eggs as recorded weekly and not because of the meter. If anything the meter let me know if the humidity was stable and when the water level was getting low because it'd drop to around 40% but that was the extent of it's usefulness.
Something as simple as living next to a body of water can change the local humidity as well.
or having aquariums in the home... lol.. people don't realize that there are so many variables.. so that's one reason why I try to get people that are having problems to monitor their eggs and try to ignore the hygrometer