Let me second what our friend Belial said. Nvidia likes to call it the "boost clock", an increased clock state that the card automatically steps into if there is thermal "budget" for that. When it happens, you will see your GPU clock quickly jump between two set frequencies (like from 920MHz to 1050MHz) for a while.
Your card does not seem to be using this, either because there is no thermal "budget" (the card is already at the top temperature at the base frequency), or because it is disabled.
My personal experience is that it is better to have a fixed frequency. The card will usually be able to "boost" only when it is not under heavy load. That means when you most need the extra GPU power, it won't be available. I also don't like the idea of the GPU switching states under load, for stability concerns and the possibility of hiccups.
While I agree with Belial that you may push your card to whatever limit available on afterburner, as long as it does not crash nor produce airtifacts, you will probably soon realise there is little use for that. I own a stock GTX 680 myself and very rare are the occasions I could use more than that.