Different hashrates when mining with the same card (GTX 980 ti)

Hi,

I'm currently mining with my gaming PC, which has a GTX 980 ti but the hashrate isn't constant. One moment it's at 16MH, the other it's at 20/21Mh. It changes between these values at random times, for no apparent reason that I can find. Here's a couple of screenshots:

http://i.imgur.com/nnKMDHX.jpg

http://i.imgur.com/1bP3UNM.jpg

You can see in the second one, where it's mining at 20, that both the card power and FB usage (which I assume is the video memory?) is higher. My question is, why does it changes between these 2 values and how can I force it to always mine at 20MH?

Thanks