Sleeper lists are everywhere...and a lot of names get more run than they deserve. Even incredibly talented players can be overrated in fantasy if they're in the wrong situation. In other cases, the player’s talent, situation and past production simply don’t suggest the likelihood of a breakout at any draft position.
Note: When a sleeper's average draft position rises too high, like Robert Griffin III's, they cannot be considered a sleeper anymore. RGIII is now (generally) being drafted as a QB1.