Drop shadow offsets underlying BMP by 1 pixel (when printed). Bug?

Hello.

I'm working on a software documentation which shall contain some annotated program snapshots. Hence I made BMP screenshots, imported them into Photoshop, changed dpi resolution so that they would fit onto an A4 page when printed and finally drag-n-dropped the PSD onto an Illustrator CS page.

Then I added lines with the drop shadow effect. Everything appears to be fine when looked at in AI but in print, the bounding box of the shadowed line does offset the underlying BMP region vertically down by 1 pixel.

This not only happens when printing the AI file with my non-postscript Laserjet 1100 but also when saving it as an EPS file.

This screenshot from a GhostView page display shows the effect:

<http://www.sterbetage.de/misc/ds_offset.png>

Again, when importing the eps into Illustrator it looks correct. Is this a bug or have I done something wrong whilst applying the drop shadow?

Kind regards, Malte.