The private keys were deliberately generated within those ranges by the author. The ranges themselves weren't calculated.
So it's like the author made #64's private key between ranges 2^64 and 2^65-1, #65 between 2^65 and 2^66-1, and so on.
why at certain times the kangaroo program start to accumulate dead kangaroos? does this give any indication as to solving the the puzzle