Construction costs are really low now. It's a good time to build if the space is in a future plan anyway.
The building's shell is nothing to Apple. They make more in interest on their cash hoard in a month than that building costs. 98+% of the "billion dollar" data center is what goes inside the building. The equipment... For example, there is no sense in installing a $800K chiller system if there no reason to chill the air. A subset of the equipment has to be installed as the building is built. It is not any cheaper to buy that equipment earlier and have it sitting largely idle not returning anything on the investment.
Likewise grid/cluster/cloud (whatever want to call it) packaging technology is changing. 8 years ago folks would have just just thrown in several truckloads of racks and just started plugging in 1U boxes. That's isn't the best way to do things on this scale. None of that makes major changes to the outside of the building but waiting till later could incorporate modifications to the interior.
Some folks use containers now. You roll in a big box and plug it in. [ Note: that is yet another reason don't want multistory building since would have to put very large boxes on elevators or need a crane. ]
Another reason to split the center into two buildings is that you can do upgrades and rehab on one portion without disrupting the other half. 3-4 years from now all the severs and tech being put into the first building will be obsolete and they will be ripping that stuff out and putting new stuff in.