If I'm not mistaken, this would be the features of the i7 at work. When it is idle, it underclocks itself to save power, and when under stress it basically goes into "turbo mode" to overclock itself. The names of both these features escape me for the monent, but I'm sure you get my gist. These features if I'm not mistaken manipulate the multiplier to achieve the altered clocks. Don't think it's cause for worry unless its hurting your performance. You can also turn them off in the bios.
EDIT: Ahh, ganzey beat me to it. That's what I get for taking so long to reply haha.