The horrible truth is that the 0% chance you saw for a white DOFF was not quite 0%. You can see this sometimes when changing the doffs you use for other missions to increase a crit chance... the blue shaded area will increase very slightly but the % chance for crit does not go up.
Unfortunately, the numbers are rounded to the nearest whole up or down.
In your case, the zero may well have been as high a chance as 0.49% (less than a half percent chance of worst outcome) but you beat the odds and got it.
I know it sucks but you can take a pittance of comfort from the fact that it works both ways and a 0% chance of crit can sometimes come in.