Well, the figures are obviously off. 20 a year is a heavy overstatement. You may be right, but I don't think spiders like dwelling in wet places. It surely is possible, just very, very unlikely. And also, I think that a human would notice if a spider placed itself in your mouth, even if you were sleeping. So, you'd need a spider that for some reason liked very wet places, and a human that wouldn't wake up. And also, a spider that didn't notice it was entering the mouth of a much, much larger (and such, far more dangerous animal). Now, spiders aren't stupid. They aren't clever, but they aren't stupid either.