Red Guardian, Natasha's adoptive father, is named Alexei Shostakov. Red Skull mentions Natasha's father as Ivan, who is, I presume, Natasha's biological father.
However, Red Skull mentions Gamora's father as Thanos, who is her adoptive father.
Why, for Natasha, does Red Skull mention her biological father as her father, but for Gamora, mentions her adoptive father?
Why does that differ?