I think itīs meant to be some kind of "Social Feint", so I can see where this idea of "failing to get a bonus later" comes from. However, because itīs still a voluntary and false fail, this really doesnīt make sense once you think it through. Itīs basically a kind of elaborated con, so why should you profit from having a low dice pool?
Well, there might a another way to read this: Itīs not clearly what happens if you win the test. If you interpret this as "Achieve whatever your con would normally achieve", this makes a a little bit more of a sense:
First, you make a con attempt thatīs voluntarily bad and likely to see through, thus the higher modifier of -6.
If you still manage to succeed, kudos to you. Your mark is convinced of your sloppy lie.
However, if you donīt succeed, your target is now in the false belief of being able to see through you, making it prone to your next attempt.