Because it was a game in her mind where she called Richtofen her father too.
Outside of her mind, Maxis is her father...
If you ever listen to all the recordings throughout all the maps, Maxis puts Sam and Fluffy into a teleporter or some sh*t then she gets teleported to some place or whatever...
Richtofen then hangs around with Maxis, learning from him, being his student and his friend...
Richtofen was just Sam's friend.
Sam is Maxis' daughter.
Fluffy is the leader of the hell hounds.
Etc etc.
Any questions you have, those recordings will answer.
Ignore ALL recordings on Origins because that was officially just their game