"Spring" is a somewhat ephemeral ride quality, and there's lots of debate on the different components that go in to that. I'll say only 2 things.
First, tire width and quality has a lot more to do with it than anything else.
Second, the boys and girls at Cannondale do have alu pretty well dialed in. Not sure what it is, but I felt a similar snappiness with my CAAD7 road bike, when compared with other aluminum frames set up similarly.