Smart baseball people will tell you not to try to evaluate trades in terms of who “won” and who “lost.” Luckily, I am not a smart baseball person.
The underlying idea is that trades are part of a very complicated matrix of roster machinations, involving everything from salary to 40-man rules to team chemistry and on-and-on. A given player may be blocked at the major league level or may be at risk of being lost for nothing to waivers. So to simply add up the WAR value of players on either side of a trade may not tell the whole story.