Is it a fresh boveda? Has it been recharged? I did an experiment a few years ago with a boveda pack that had been recharged to the point of being very plump. The RH readings I got with this pack were significantly elevated.

If it a fresh, of normal thickness pack, it is possible the hygrometer could be that far off, but I would verify against another Boveda pack before I go adjusting it. Actually, I would probably do a saturated salt test after checking with another pack; but I won't suggest that since too many variables and not many people have success with it.