Black bars when downconverting HD to SD

I am trying to use Media Encoder CS5.5 downconvert a HD clip (1920 x 1080) to DV PAL anamorphic (720 x 576) .

However there only seem to be 2 choice for me in the export menu in terms of pixel aspect ratio.

By selecting D1/DV PAL, the exported clip is a 4:3 letterboxed video - which is not what I want.

By selecting D1/DV PAL widescreen, the exported clip is an almost-correct anamorphic video.

I say 'almost-correct' because there are narrow black bars on the left and right side of the exported clip.

The image below shows the preview of the exported clip (with black bars at the side)

Does anyone know the reason for the black bars at the sides? And how can I get rid of them.

Thanks.