Most cars have a black ceramic 'frit' edge on the rear window. Sometimes this ceramic edge is straight and flat, sometimes it will have a dotted edge. These black ceramic dots are quite thick on most cars and prevent the film from sticking to the area between the dots. This results in a whitish looking strip trimming the glass.
Typically this isn't very noticeable, however some cars have a wide band of dots trimming the top of the rear window. This area is so wide that the whitish looking area created is more pronounced.
http://www.tintdude.com/dot-matrix.html

http://www.bimmerfest.com/forums/archive/index.php/t-95832.h...