What do you mean by the difference is small? In absolute value, it is  
small of course, but that does not matter!
What matters is the expected number of fires for a given CDF, which will  
be exactly 10 times as much with a value of 7 than with a value of 8 (if  
the probability model is appropriate).

-math.log10(1.0 - cumulativeDistributionFunction(timeDiff, mean,  
stdDeviation))

This is a logarithm of a tail probability.