The trade Snead made in 2012 was a mistake that he readily admits: the Rams traded down 5 spots when they only had two guys they coveted - Wagner and Mychal Kendricks. (Side note: Kendricks was drafted before Wagner.)
In the 10 drafts since then Snead has applied the hard lesson he learned in 2012: Don't trade down 5 spots unless you have a group of at least 6 guys that you would be equally happy drafting. So in 2019, when the Rams traded down from 45 to 56 the underlying piece of information is the Rams had a group of a dozen (or more) players that they had the same draft grade on. Essentially, they knew they'd be able to draft a guy at 56 who had the same grade as the guy they could have drafted at 45. So why not trade down?
We can of course argue about how well the Rams grade players. The point, though, is that the Rams believed that they lost no value in the player they picked by picking at 56 rather than 45. Heck, for all we know if they had no trade partners they might have just picked Rapp at 45.
AlbaNY_Ram