That is odd. I've heard that ideal ranges of humidity for BPs isn't high enough to cause condensation. Who knows. If your hygrometer says its ok, I wouldnt worry too much right now. Maybe you could buy another one when you get a chance, just to verify the humidity.

Is your water dish on the side of the condensation? And is it above your UTH?

The only problem with condensation is bacteria, and it sounds like that wouldn't be a problem being as it's only a small amount. Just keep the tub clean.