Not too hard to see what the plan is here.
CapGeek, again
First off, as a young up-and-coming player, it makes business sense to not lock yourself into a long-term deal, when you can expect yourself to play into a raise. So, good move on Leddy's part. He could be a whole different player in 2 more years.
Second, this makes a lot of sense for the Hawks, too. Hjalmarsson's contract is up in 2014, and will likely be getting a raise (here or elsewhere) if he continues to play the way he did in 2012-13. The key here, though, is that Oduya's contract is up in 2015. He'll be 33 years old then and will most likely walk in free agency, if he's not traded before then. That takes $3.38 million off the books and the salary cap is expected to start increasing as the league expects to make more money (duh) in the next few seasons. Gives the Hawks the ability to throw some more money at Leddy, should he continue to develop as a hockey player.
These are the kinds of subtle little deals that teams make to achieve long-term financial stability.