It's not unnatural to have one slightly stronger arm, it's bound to be the case as you will always use one more than the other. If you want to even them out though, do not start lifting more on one arm, or just focusing on your left as some people have said.

Best bet is to stop upping your wait for a month or so, and simply focus on perfect technique. The uneven look usually comes from upping the weight your lifting too fast, though you may feel you can lift it - the way your muscles develop tend to make it look a little odd.

But hey, biceps, who gives a fuck? Probably the least frustrating place to be uneven!

Are you right handed? That's probably why. Try lifting some weights more with your left arm and see if that evens them out.