Tis when it comes to standing I just had a debate about this in one of my poli sci classes. A lot of men and women see height as strength. So for Bush to be standing next to Kerry it would look to some (sub conciously) that Kerry is the stronger and more fit for the job kind of thing.
With them both sitting they appear on the same playing field.

One of the debates between George H. Bush and Bill Clinton they were both seated