It's because the evidence turns out to be real (ostensibly), they just fabricate how they obtained it. They get information which leads to an arrest. Once the

It's because the evidence turns out to be real (ostensibly), they just fabricate how they obtained it. They get information which leads to an arrest. Once they get the arrest they construct an alternative explanation for how they got the original information in order to avoid scrutiny about how they originally got the information (such as the Sting Ray). The information they got from the Sting Ray is, ostensibly, true/actionable but the Sting Ray itself is questionably un-constitutional. So instead they create a fake evidence trail leading to the information they got from the Sting Ray in order to explain any and all information actually obtained from a Sting Ray. Like someone else said, it's kind of like evidence laundering. "How we got this is bad, so instead let's create a fictitious string of events to add legitimacy to it."