Because perf variability can be 50% according to Intel in fonction of the device temperature, i guess it was better to launch them on december, but anyway the TDP is temperature dependant, 4.5W is for a heated device that run will run close to 1-1.4GHz.
Yea I'm sure that's why they are launching these devices in the winter for the northern hemisphere. Maybe the southern hemisphere will have to wait till July 2015 and along the equator the release is postponed indefinitely.