I obviously don't work for ANet, but I do work in the industry. My guess (this is just a guess for the sake of discussion) is that when they planned out the story for EOD, they weren't sure how it was going to sell/be received, and they left it at a spot where if things didn't go well financially, they could leave the story and it would be complete. But, now that it seems to have gone at least ok (maybe it sold better than they'd wanted, I have no idea), then they're comfortable saying that there will be more without actually saying what that more is.
