My guess is Rapp isn't the only guy they had their eye on when the Rams traded down. There could have been a half dozen guys they would have been happy taking at 31, not just Rapp. So they traded from 31 to 45, confident that at least one of 'their guys' would still be around at 45. And when a bunch of them made it to 45 they traded to 56, and again to 61. Rapp might have been the last guy left from their original group when they picked at 61 (rather than being the only guy the targeted).
AlbaNY_Ram