Maybe because it's like 30 minutes shorter than the first soundtrack. Sawano says it's often difficult to include all the songs on a single soundtrack but he is sometimes able to release the extra tracks on a bonus CD like with Guilty Crown.
Samples can be heard here:
http://visual.ponycanyon.co.jp/picku...al2/pcxg50274/
Edit: Oh, and going by the recording date in the track titles we can figure that 2,3,5,6,7,9 are tracks that they weren't able to include on the first soundtrack and 1,4,8,10,11 are the additional tracks that he was asked to write later on.