I just talked to one of my sources and as it turns out the NBA teams have several trades that are worked out before hand with different senarios that just never happen on draft day. Apperently the Spurs are always trading up and down and it just doesn't happen for one reason or another. For example, say the Spurs wanted player X. Team A is only willing to trade and draft player X for and to the Spurs if a couple things just fall the right way. So if the Magic really like Courtney Lee and not much else in the draft and Courtney Lee is drafted ahead of their pick the Spurs might could have offered something so they could get the Magic's pick to draft Batum. These kind of deals go down all the time, but more times than likely a team like the magic have 3 options for thier pick, and only if all 3 of those options are not avilable will they trade the pick to the Spurs, and the Spurs will only pull the trigger on this trade if the player (like Batum) is still avilable at the pick they were trading for.
It is very complex and hard to keep up with, but the Spurs did it last year with Hairston and Dragic. The Suns like Dragic, but figured that someone would draft him in the time between pick 45 and 48, so the Spurs would only offer that 45th pick if the player they really wanted at 4 was not in the draft pool or they figured that the player they wanted to draft at 45 would still be in the pool at 48, and the Spurs get some bonus for protecting Dragic for the Suns. Like I said it is all complex.