I don't think Bayes' theorem applies very well here, because there's no practical way to check enough numbers such that the effect you're describing comes into play. 2^80 is approximately a million billion billion (10^24), so if you had a million CPUs each checking a billion numbers every second, it would still take a billion seconds (over 30 years) to check that many numbers. I suspect that's why such a small level of uncertainty was chosen in the first place.