'Fields are filled in with (unknown) after importing csv using fread in pydatatable

I have a csv with about 15 columns, this file is imported using fread from pydatatable.

dt.fread('temp_sample.csv')

Here some of columns are contained empty values i.e NAN's and partial entries and some other columns are completely NAN's.

Here country column is filled with (unknown) as it has no entries and whereas SSN has got NAs and entries existed.

Why (unknown) is filled in in this type of cases? Why it is not as NA's?

python py-datatable

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