It's not entirely clear through the playscript if Shakespeare meant the nurse to be purposefully hiding the identity of Romeo from Juliet, or if she's simply being absent-minded, as she's portrayed most times. I'd like to think it was up for interpretation, and I believe this way (her purposefully mistaking who Huilii was referring to) adds a little bit of duality to Kelara's personality. She wants Huilii to be safe, but at the same time, she wants her to be happy.
It adds some conflict that otherwise would not be obvious if not explored.