Everyone has given pretty good responses here.
The "Variance" is a measure of how well your CPU is responding to game requests. If it is not flat, then it normally means something else is interrupting the CPU and stopping game requests from being satisfied.
Anything can cause it. Errant network requests (some games install a bittorrent client which can eat up the CPU if not properly configured, iTunes is also bad about this, along with any other P2P progam). Malware, spyware, anti-virus software, anti-xxx software of any kind (Norton and McAfee are not friendly at all about this). That is just a couple of things of the hundreds of things that can cause it.
Onboard video chips can cause it as well as they have no dedicated video RAM, which causes the CPU to have to stop every time something has to be drawn.