How are missing months of birth assigned in the 1900 full count?

Hello, I couldn’t find this documented anywhere about the 1900 full count census so apologies if it’s already been answered. I know from examining the census manuscripts that there are often cases of people with their birth month recorded as “Un” or blank, but in the downloadable sample, there are no cases of birth months that aren’t the 12 actual months. So were these cases all dropped from the sample? or were they filled in somehow?

Thanks for any clarification!