ButtercookieIt's a step up in materials, and in a way replaces the SB20. The most notable difference is the use of 800 fill power down, instead of 600 in the Cosmic 20. This is more expensive but allows the sleeping bag to compress more, and therefore take up less space in your pack. The Sine uses a lighter, thinner 20d fabric instead of the Cosmic's 50d. The combined weight savings allows Kelty to offer the unique spiral zip for foot venting.
ButtercookieAs I understand, the fill power is 800 that helps with the weight (only 1kg) and as dannymiks said it allows the sleeping bag to compress more.