There's something called the eye test. They have both been in the league for three years. One is a year younger than the other and has shown marked improvement in multiple areas of his game. The other, a year older, has shown absolutely zero improvement in any area of his game and has bounced around from team to team to team (now on his 6th NBA team).
Since you like stats (when they serve your purpose), consider this: Meyers Leonard put up a PER = 19.6 in the playoff series against Memphis, who has one of the best front courts in the entire NBA. That's an improvement in PER of 27.4 over his previous year's playoff performance. Yes, this is a small sample size (106 minutes), but what, exactly, has Thomas Robinson accomplished in a similar sample size (122 minutes) of post season play? Anyone who can't see the improvement Meyers Leonard has made has his head firmly and stubbornly buried in the sand (or some other dark place).
There's a reason 5 teams have given up on Thomas Robinson. He has shown no significant improvement in any aspect of his game. He still relies on his athleticism (admittedly, he's very impressive athletically) to make an occasional spectacular play block or dunk), but is still the same fundamentally flawed project he was when he was drafted. He blows defensive assignments, doesn't know the offensive plays, doesn't have a single low post move, makes stupid, unforced turnovers, commits stupid fouls and can't make a FT to ave his life.
Does Meyers Leonard also blow defensive assignments and commit stupid fouls and turnovers? Hell yes (but at least he makes his FTs), but he has shown improvement in these areas. During his first and second seasons, looked like a deer in the headlights. During his third season, and especially the playoffs, Meyers Leonard actually looked like he belongs on an NBA court.
Those people who so quickly discard one, include 5 NBA GMs who have traded, cut are let him walk. The person's opinion that matters, has decided the gold mine was worth hanging onto. So far, he looks to be right.
Bottom line: Meyers Leonard is a year younger and has shown significant improvement. Thomas Robinson has not. That's why one's stock is trending up and the other's stock is trending down.
BNM
