2004 Taurus slight RPM fluctuation while at idle?

My wife and I were test driving 2004 and 2005 taurus's this past weekend. All the ones we test drove were either the SES or SEL with the 24 valve 200hp V6 engine. We noticed that sometimes, at a stoplight, when you first start it, or right before you shut it off, basically at idle, the needle for the rpm's would fluctuate. The needle wouldn't move much, it would just kind of hover/slight bounce around 1,000 rpm's. It didn't happen on every taurus, but it did happen on more than one and it was not consistant on any of the ones we drove. At first I thought it might have been the timing that was off, but then I realized that these fancy new cars have computers or sensors that control the idle and you really can't adjust it anymore. I am not sure what this could be or is this just a newer taurus thing? Any help would be greatly appreciated. Thanks in advance!