Hi Eric,
The BER pattern defines an area such that any decision made inside of that area (decision amplitude and decision time) should provide a minimum BER of 1E-12, 1E-11, etc. (defined by the user). The calculation (which is statistical – complementary error function – erfc) is based on the average power of the 1s and 0s, the associated variances of 1s and 0s, and the amplitude threshold value. The time threshold defines the average power and variances to be used for each calculation point of the pattern. Since this changes as you traverse the eye, the pattern will not necessarily be uniform (especially if there is distortion). In general the BER pattern will be centered around the middle of the eye as this provides the highest difference for P1 and P0 average values.
As shown in your diagrams, the BER patterns occupy a much smaller region for the 100 km case vs the 10 km case. This is due mainly to signal distortion from dispersion and a higher susceptibility to noise from more attenuation.
I hope this makes sense. Cheers