I can't believe I did this --- but I checked this out. It turns out that the sensor in the side for the one piece only is tied to the level of toasting you want; it checks for how brown the bread is. Now, this opens a whole question of whether white and wheat bread toast to different levels of brown, but that, my friends, is a question best left to the toasting professionals.
Here's a site that explains the toaster in engineering terms.
Link