Bees actively dry honey.
Usually over-night.
Have you been in the back yard when it's dark out, and smelled the
honey smell from your hives?
They're pushing a lot of air through the frames, and pushing the
humid air out of the hive, along with the aroma.
If you Harvest your honey in the later part of the day, the day's
nectar will be distributed through the hive
and some of it may be in open cells in he frames you have your eye
on.
If you harvest before the bees get busy foraging in the morning, the
honey in open cells will be mostly dry/cured/ripe.
I agree with the shake-out test. If you can't shake a sprinkling of
droples onto the hive, then the frame is probably harvestable.
It's a dilution issue. A few pounds of finished honey can't pick up
that much excess water from a few ounces of not-quite-there honey.
Nevertheless, I prefer to harvest frames that are completely capped
or mostly capped.
A refractometer is a good investment and they're not expensive. GET
ONE DESIGNED FOR HONEY OR SYRUP. (not one for brewing or antifreeze
etc)

a "BRIX" scale is harder to use than the %-water
scale.
Sometimes the line indicating reading in the refractometer is
sharp. That's because the sample on the prism is homoginous.
When the line is broad and indistinct, the sample is not uniform.
It has lesser and more dried out honey in the sample.
Get a different sample, or mix the honey and try again.
that's my 2¢