auto_now_add=True with null=True

I have modified on my model by adding a DateTime field as the following

date_joined = models.DateTimeField(auto_now_add=True,null=True)

when I migrated on the previous rows, this column is set to now value not null as the following from the admin page.

Date joined: Oct. 9, 2019, 1:32 a.m.

any explanation please ?