This plugin doesn't specifcally choose the ebook editions, it will use whatever edition you have linked to. That link may have come by using the Goodreads metadata source plugin, or by the user manually linking using the Link to Goodreads function. You can switch editions using that same dialog.
You might get the ebook edition at times, either if the ebook edition had a unique isbn (and you had used the Extract ISBN plugin or manually populate it in calibre) or if by pure coincidence the matching logic on the title/author search results picked the ebook edition as the first matching result.
But there is no logic which dictates the ebook edition must be chosen. If the quality of the data was sufficient out there to guarantee the ebook edition existed and had all its fields meaningfully populated then it would make sense to do something like that. But as you have found yourself the data is frequently lacking in that department, so users like myself are happy to just get an edition with sufficient data on it to be useful (such as having a page count for the Goodreads stats page or Count Pages plugin purposes). Frequently the internal logic Goodreads has to order the search results means that you get the first time, but other times you have to switch editions to pick the one you want.