I just had to check... My 6805 has AMD driver 1.1.0.0 and Power Now for Win2k installed in compatability mode (Win2k), using original load of XP home, and I can use Low (800 Mhz), Auto and High (1.8 Ghz) in either Battery or AC. If Power Now installs right, you get a Power Now tab on Power...
It's probably Cool and Quiet or Power Now at work. AMD's throttle down to save battery. Tons of info available Here. Search for Power Now, you'll probably find an answer.