First time I had doff refugee all 3 traits green and amazing percentage on blue and purple.. and i get rounding error... lucky me!
The rounding error is in the display, not the actual percentages.
The datatype on the percentages is probably a long int or equivalent, which means it can store quite a few digits. So the actual percentage might be 0.04522389214% fail. There's no way its going to display that in the UI. So it rounds to the nearest (0%). But its still a 0.04522...% fail chance, so every so often it will fail.