I always do this too, and i think it is correct after the tap. It tends to "stick", and the amount that it jumps depends on the change in humidity since i last checked it. I had a reading once, in a new, dry humidor, of 50%. I put distilled water in the box and left it for a few days. When i opened it, the hygro still read 50%. When I tapped it, it jumped to 72% ! So I know for a fact that mine sticks, and requires a tap to be accurate. I suspect yours is doing the same.
