In the oblique case explanation for postpositions, it says this:
का/की also influence the preceding entity to take the oblique case.
Can someone explain this to me in a simpler way?
It's actually similar to the way it's done in spanish. 'Ka/ki' decide the gender of things like boy=lad'ka' , and girl= lad'ki'. Similarly, his laptop=us'ka' laptop and his book =us'ki' kitaab(book). This indicates that these objects re also assigned gender so are pronounced in that way. If you have anymore doubt or I misunderstood your question, feel free too ask.