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This happens more than you'd think, and it can be frustrating. It can, on occasion, be a sign that Google is on the fence about your category page. In other words, it could signal quality issues, although that's not always the case. Could you tell me what search you were running when this originally happened? There are ways to see how Google is viewing the pages on your site that match that search.

The conversion of the ebook download and the conversion to the product sale being on the same page put the two objectives in competition with each other on that page and can make it hard to segment your visitors. Link building to the ebook conversion page will certainly help the authority of your product conversion page and entire domain.  In fact, using an ebook page as the basis of many successful ecommerce link building strategies.

Had a bit of a dig and found this post which seems to confirm the reference being followed such as it is although not clear if any real ranking benefit. http://dejanseo.com.au/seo-experiment-with-non-link-references/ Certainly is an interesting case and I agree nice to be able to 'fix' these errors and if there is some seo side benefits of any kind, so much the better!