I think the preview, wishlist/recommendation item, archived book, and full book are all the same object as far as the Kobo device database is concerned. They just have different tags to show whether the object is in the recommendation/wishlist state (not purchased, not downloaded), the review state (not purchased, partially downloaded), archived state (purchased, not downloaded), or full book state (purchased, downloaded).
I think something changed in firmware 2.4.0 (or maybe 2.3.1) and there are sometimes issues because if this. I find that if I buy a book that I have a review for, the file size assigned to the book is the size of the review text, not the size of the full book. Similarly if I buy a book from my wishlist/recommendations then the filesize assigned to the full book is tiny, probably the size of the metadata (synopsis etc.). The only way to fix this seems to be to edit the sqlite database to set the correct filesize, or log out of my account to rebuild the database. Both are a bit of a pain, so I just don't use the wishlist anymore and never buy anything that is in my recommendations.
Instead I maintain a wishlist on
http://luzme.com. This has the advantage of showing prices for te book in some other countries, so acts as a bit of a sanity check on price.