If Polonius knows his Bible stories, he will likely have worked out where Hamlet is going with all this Jephthah stuff, given that Jephthah’s primary story is about his daughter. In that case, he’d be baiting Hamlet a bit, waiting for confirmation of his thesis that Hamlet’s gone crazy with love for Ophelia. If Polonius isn’t so quick with biblical allusions, this mention of daughters in the line before might indeed be a surprise.
Polonius would have to have some confirmation bias going as to whether or not Hamlet’s talking about Ophelia. Polonius is primed to hear references and Hamlet aims to give them. A perfect match.
The Hamlet Project 