In Chapter 5 of the novel — Why did neither of the Rosens tell Deckard that Rachel was an Android until Deckard figured it out, when it would’ve benefited the Rosen’s upon the test’s initial results? & Why did they bribe him with the owl upon informing him of the test being recorded and for what purpose?

Could someone please clarify this for me? As I’m having a hard time trying to grasp the answer of these questions with how the wording of the book is.