The Star Wars EU has established (for their canon) that a ship making a jump to hyperspace under scrutiny can have its destination known. Hyperspace isn’t (usually) like some sort of warp drive that you can ‘drop out of’, it’s a ‘we are jumping to these coordinates’ thing, and a ship watching with the right instruments can jump right after them. Now, this is bolstered to a certain degree in this film by having the Death Star appear to jump to where the Millenium Falcon jumped. You note that they have to fly around Yavin to get to the moon with the base, and likewise, the DS must do the same.
But then… we have the dialogue which describes a ‘homing beacon’ on the ship. If they had a homing beacon, why wouldn’t they just jump right next to the moon that the dang Millenium Falcon was sitting on? Or was it a beacon that just gave, say, the hyperspace exit coordinates or something?
If the homing beacon really worked, then whoever was at the wheel for the DS must have been in some pretty serious danger of getting written up for not jumping into attack range.
Unless… let’s see… unless the DS needed time to charge up the Superlaser following their hyperspace jump, and consequently chose to jump where they did for strategic reasons… Yeah, let’s make that the reason for this. 
BTW, did Alderaan have much space traffic? How many ships would be flying around that system when everything went down? Anyone there have a camcorder? Or would “stunning footage of the terrorist stronghold of Alderaan being neutralized” be something the Empire would want out there? I guess that would fit with the ‘Tarkin doctrine’ alluded to by Mr. Cushing.