Yeh windows isn't actively measuring the speed, it just reads what chip it is and states the stock speed.
And it doesn't always do that, I replaced my nans core 2 with a q8400, and it took a few reboots for Windows to notice it had magically grown two more cores.
It still ran as expected though so don't worry about it. Hwinfo or cpuz will let you know what's really going on.