There are good reasons for different players having different lengths on their contracts. Both Stephens and Dunk have more stock value so if sold with years on their contract we get top bucks. I think Bong is a good LB but he has had lengths of time out over 2 seasons with I believe the same...
I guess the club have to consider his injury form. He does what is needed on the pitch and if he was never out injured his contract offer may have been longer.