Looking more closely at the BIOS...on the "Current Status" Page, it says Non-Turbo Frequency: 3.404 MHz. Turbo Frequency: 3.804 MHz.
So CPU-Z must be reporting an average between the two. And Windows is reporting the higher of the two.
Huh?
For the record - this is all a bit new to me, particularly how turbo works. Came from a Core2.