Aperture, Preview and Photostream

When I put an image on the photostream using Preview, at a later time I can still see it in Preview.

When I put an image on the photostream using Aperture, at a later time I can still see it in Aperture.

But not vice versa, picture put on photostream with Preview are not visible in Aperture, and pictures put on photostream with Aperture are not visible in Preview. Why is this? After all they support the same kind of documents.

Koen