It was most likely a continuity error. That is, the team who made 6 didn't do their research. The existence of another nest or Ms. Swimmer adopting more children since 2 are just mere speculations on my and Dr. Curzon's part, with no evidence to back it up whatsoever. But it's alright. Here on the GOF we speculate all the time. Some discussions might yield un-expected results or new ideas, and even inspire fanfictions. So, it's not all over-analyzing for over-analyzing's sake (sometimes it is, though; we're having fun here as well).