The new i5-580M processor

Apparently there is some difference between the system bus between this particular model and all the others in the i-family (4.8 GT/s vs.
2.5 GT/s ).
Does any one have a clue why?
And do you think that it (2.66-3.33Mhz|3MB L3) will generate less heat/use less juice than the i7-620(2.66-3.33Mhz|4MB L3) and the i7-640(2.80-3.46Mhz|4MB L3), considering all three are part of the "new" 32nm line, have integrated GPUs and the only difference I can see is the system bus in the i5-580M?
Thank you in advance.