It has to do with playing level vs expected playing level. I expect Greene to be a top pairing D-man on this team. I expect Zidlicky to be solid on the PP. They consistently do that.
I expect Clarkson to score 15-20 goals a season. He just hit 30. I expect Tallinder to be a traffic cone. In the SCF, he looked like a decent hockey player.
The latter two are the types you trade. GMs care more about what the player has done recently. When they see the tape, they'll see those two guys better than they really are, and that's how you get maximum value out of a trade. If the players keep up the "over their head" pace, then you got fair value. If they regress to normal, you got a steal. Very rarely does a player play above his normal abilities, get dealt, and continue to improve. I guess someone like Andrew Ladd would be an example. Most GMs know where their own guys should be.