Why do people always say (insert tall player) is a sub par rebounder? One, they're going up against people their size and two, you don't rebound with the top of your head, which is why length is generally more important than height.
Bertans is 6'10'' in shoes, with a 6'10''.25 wingspan. Murray is 6'5'' in shoes, with a 6'9.5'' wingspan. Anderson is 6'8.5'' in shoes, with a 7'2.75'' wingspan. That's one reason they're good rebounders and he's not. The other is, though they're all string beans, the difference is, they don't have to contend with bigs (generally, in the latter's case). He lacks both the strength to box most out and the length to corral the ball.
Also, the fact that he was the 42nd pick and is a legit NBA player already ensures he's not a bust.