It's clearly due to the coriolis effect which asymmetrically couples to the delta "T" in altitude, compounded by the phase of the moon and random planetary alignment. Nail your humidor to the ceiling and all should be well.
My god, man, this just isn't rocket science. If you ask a large enough bunch of guys what the "right" RH is, you'll get answers all the way from 55% to 75% so freaking about over a few points of RH is ridiculous.
Beads aren't some microprocessor powered magic humidity control device, they are simple yet very effective BUFFERS of humidity. That's why the volume factor plays into how many you need. So, up front, if you don't have enough beads for the volume you can't expect things to be stable, long term. Getting more than you need will keep things stable, longer, with less maintenance. The converse is that if you don't have enough beads, you'll have to fiddle with it more.
Too dry? Put a shot glass of distilled water in the humidor for a few hours, maybe overnight. Longer as required. I know in our home when the forced air heat kicks on the RH in the house will drop to 45-50% when it's cold outside. With my old desktop, I'd put a foam element humidifier in there wet with distilled water and leave it. Worked like a champ.
Too moist? Leave the lid or door open for an hour.
Flapping the door open and closed also won't help. If you make a "change" to the environment, shut the dang door and leave it shut for a day to see what the effect really is.
You are NOT going to wreck your cigars if you have a short term excursion in RH. I was recently playing box tetris with my cabinet and managed to leave a box out for a couple of days. The smokes were fine and all's well, long term. No panic, no worries, no big deal.
Of course, if you can't count on your hygro, this is all a moot point. Salt test your hygro with fresh batteries and record the delta if you can't adjust the hygro.
B.B.S.