A number of reasons:
Google plays around with the format. (for there own placement research)
Main reason though is dependent on how many ads are run, for example 10 sites could run adwords, but have a daily spend of £5.00, once this is spent there ad will dissapear, this will result in the format changing.
Depending on your click through rate, your ads will move up or down without changing your daily spend.
(the below used to be true, but may have changed)
Top sponsors (at the very top) are normally reserved for high paying ads, but if there are no high paying ads, or your click through rate is good you may also appear at the very top sponsored listing.
Hope that helps your Boss ;)