Comparator input offset voltage
There are two parameters in a Normal Distribution,  the mean and σ, the standard deviation. 

The mean here is +3.0 mV and the standard deviation is shown for the probability of a logic 1 output.  The inverse curve would be the probability of a logic "0" output.  

This Normal distribution curve of a transition or the "Bell curve" would indicate the probability than any threshold transition would occur.


The Cumulative Function of the Normal distribution is the integral of the area under the curve across the range of inputs was what was actually shown.

ref Wiki