Why did Luft struggle with the test in Do Androids Dream of Electric Sheep?

In Do Androids Dream of Electric Sheep?, the bounty hunter Rick Deckard is working through a list of androids to retire. Luba Luft is one of those androids. She resists the test because she does not want to be found out and destroyed.
When Deckard finds her at the opera house where she is a singer, he tells her that he is there to administer the Voigt-Kampff Test, which will determine if she is an android or a human being. Rather than agreeing, Luft takes the offensive position. She not only insists that she is not an android but also challenges Deckard over his human and policeman status and suggests that he take the test. He refuses and begins testing her. Again, she goes on the offense, this time by asking questions instead of answering them. She finally manages to knock the machine away and, further challenging Deckard, insists on calling the police, ostensibly to verify his claims.