I don't know I'll guess:
He couldn't spell one of them so he double up on josephs offspring?
Or maybe he wanted it to be an even number and 13 was considered unlucky and wouldn't divide in to 144,000 to nice and they hadn't developed decimal pionts yet as in 144,000/13=11076.9231 or maybe because 12,000x13=156,000 not a very pretty number symbolically speaking.
He was running out of room on his papyrus?