In fact in the 18+ months I've worked with AdWords, I don't think I've ever encountered a single keyword for which there was only one advertiser. One advertiser showing, perhaps - but always with several others waiting in the wings.
As vibgyor79 pointed out, you may well be the only advertiser to show in a particular moment, but 'behind the scenes' there are other advertisers in a queue to show when their daily budget allows for it. And when their ad does show, your CPC and position vary relative to their Max CPC and CTR.
AWA