Humidor behavior

[danm.528i]

I have noticed an odd behavior with my humidor. I've had it for a couple of months, and have only recently noticed fluctuations in humidity levels. It varies between 68-71% over the course of a few hours, for no apparent reason. It has a couple of glass windows on top, so I don't have to open it to check, and I've had two digital hygrometers side by side, with the same readings, so I know it isn't a faulty hygrometer. Room temperature stays about 66 degrees.
The only thing that has really changed is the number of cigars being stored. It s supposed to hold 100 cigars, and I've recently added 25 to bring it up to a total of 75.
I am not really concerned, just curious if this is normal.