As icores are newer, they have some improvements in:
- execution units (different Intrustion Per Clock values)
- smaller transistors (thus lower power consumption for the same work)
- movement of the IGP and memory controller (thus eliminating the MB northbridge)
The last point is important as it reduces the electrical path (so it can be done faster and with lower energy) and also reduces overall power consumption, as 2 chips will consume less than 3 doing the same work.