For every class, the number of major glyphs exceeds the size of the background by four to six glyphs. On the other hand, the minor glyphs have a huge unused area.
BTW I am using Firefox...I don't know if it makes any difference, perhaps an IE user can verify.
edit: I checked in IE, and the black background extends to the bottom of the glyphs, but the gray border ends at the same position that the Firefox background ends.