That's not really the answer, though. Regardless of hybrids, the odds of getting a copy of your breeding partner's pet ought to normally be about equal to the odds of getting another copy of your own pet.
Maybe there's something weird about the siren, but probably, it's just bad(?) luck. You can flip a coin and get heads 20 times in a row. It doesn't mean that there's anything odd about the coin, just that once in a great while you get a long run of the the same result. The odds for each individual coin toss are still 50/50.