I think what they do is wait until there is a batch of players to be added, and add them once every couple of weeks. They were probably due to add guys like Haren, Henderson, etc and since Harden was called up, they added him in too so everyone wouldn't have to wait another 2 weeks for him. It shouldn't be a big deal because most leagues have 2-3 day waiver periods, and anyone paying attention should be aware that he'd been added before someone else with lower priority gets him. It's for that reason alone that I make a habit of checking the player adjustments daily.
I think the ideal situation would be for Yahoo to just add new players that are called up every day when they are called up, just like they do for DL moves and position eligibility. At any rate, it's a free service so you can't really complain that hard.