And also, there is a difference between the word אשר when it indicates an attribute of a subject and the word כי when it indicates a future action:

And so any place where it wants to clarify that a matter is only applicable from here and onward – such as with צרעת (leprosy) and זיבה (issue), that the צרעת or זיבה that is before the speaking does not have a status of “unclean” – the word כי will come instead of אשר everywhere. And in a place where it means to say that even if this attribute were present long ago the judgement will be applicable – as when it is written אשר יהיה בו מום (Whosoever he be of your seed throughout their generations that has a blemish)²¹² – the word אשר will be used.

Leviticus 12:2 says, דבר אל בני ישראל לאמר אשה כי תזריע וילדה זכר וטמאה שבעת ימים כימי נדת דותה תטמא (Speak unto the children of Israel, saying: If a woman be delivered, and bear a man-child, then she shall be unclean seven days; as in the days of the impurity of her sickness shall she be unclean), using the word כי. According to Rule 104, this indicates that if a woman delivers a boy before Moses tells this law to the children of Israel, the law will not apply²¹³.

footnotes: ²¹² Leviticus 21:17. ²¹³ Malbim, HaTorah v’HaMitzvah Tazria 2.