When you shoot an item through a nether portal either by throwing it or by using droppers / waterstreams, they will always pop out in the +X direction on the nether side, no matter in which direction they were thrown in the overworld.

This can be reproduced by placing a portal along the Z axis and throwing an item through towards -X. When going to the nether (with a full inventory or through another portal as to not pick up the item), the item will be on the +X side of the portal.