Is the water bowl near the heat source? My gut is telling me that's probably the cause, since you say it's a decent size bowl.
Thick bedding would hold moisture and therefore up the humidity if you were misting it, but since you aren't, then I'm not sure it makes that much of a difference (though I could be wrong).
I'd do a little experiment if I were you - take the bedding out of one tub, replace with newspaper, but leave the water in. In another tub, leave the bedding, but take the water bowl out.
Check the humidity tomorrow and see what the results are and if anything has changed in those tubs.