The incentives for the reviewers are different. On-line reviewers (often) get caught up in review-counting and status-seeking. They want to be a "Top Reviewer".

The incentives for the reviewers are different. On-line reviewers (often) get caught up in review-counting and status-seeking. They want to be a "Top Reviewer". In a store the person has to go out of their way every time they recommend the same book. It's one-to-one vs. one-to-many. There's more of a chance they really do think highly of the book, maybe even years after reading it. That's valuable.