Is this a BUG or a twist in story ????????

In some remote village of India, one masterji is teaching the Mahabharat
katha to a class. He is at the 'Krishnajanma' part of it.. So let him
continue instead of me. Masterji : Dear bachcho, so Kansa heard the
akashwani that his sister's 8th child is going to kill him. He was furious.
He ordered to put Vasudev and Devki behind the bars. First son is born, and
Kansa kills him by poisoning. Sencond one is born, and Kansa thorws him off
the mountain peak. Third one is born...Now Ramu who is the smartest of the
lot puts his hand up.

Masterji (nervous & confused) : Ramu bete, whole India does not have doubt
in Mahabharata and how come u have one? Ramu Beta : Masterji, if Kansa knew
that Devaki's 8th child was going to kill him; WHY THE HELL DID HE PUT
VASUDEV AND DEVAKI IN THE SAME CELL??