I think it's because
1st. Her mother was poisoned by the empire and her hair color gives away her identity as the promised child had they revealed her real father because they would have connected the dots that since the "uncle" Went on war so long he had the chance of meeting the promised princess and make a child
Also her mother was kidnaped and came back without her child which means she knew empire would have harmed her and her daughter so she "went missing" And gave her and "uncle" Daughter to the count family to protect her
This might also be the reason why they treated her so kindly
This contains spoilers.
So her mother totally is the older promise princess. Like red hair and all. It makes since in the beginning when they called her a wandering tarot reader (something like that). I kinda wish they introduced more subtlety then head-on. For a bigger plot twist. Now I wonder if her father really is her blood born dad.