It may be that your TV is doing one of those "smart zooms" - stretching the picture out at the sides so it fills the screen (the black bars at top and bottom being part of the picture) but keeping everything almost right in the centre. The downside to this method is a bit of distortion and the loss of some picture at the top and bottom (not a problem in this case because it's black). I've never come across a DVD player that turns a 4:3 source into a 16:9 picture. TVs tend to use either the zoom mode you set, or proper widescreen if it receives the switching signal.