I suspect that it's because 95% and 99% are familiar numbers from a statistics class, corresponding to 2 and 3 standard deviations in a normal pdf respectively.

I suspect that it's because 95% and 99% are familiar numbers from a statistics class, corresponding to 2 and 3 standard deviations in a normal pdf respectively. In statistical tests, most answers have something to do with standard deviations of the normal distribution, whether it be what 'significant' results are, the error bars on a histogram or the choice of error range on a maximum likelihood fit that has no obvious correlation with normal distributions. (All of these are prevalent in the high energy physics community.) Statistics are very often used to support 'gut' instincts like that without necessarily understanding the underlying meaning of the mathematics. Happily, it is often the case that approximating everything to a normal and using 'sigmas' is enough to get by. (May not be the case here, but I deal with it every day at work... </rant>)