Depends what they mean by Productivity. Other things can be productive other than Labour. Also it still doesn't mean you should be paid twice as much.
If I can hire 3 unskilled workers for less than the price of a skilled worker that is twice as productive as a single unskilled worker. I would just hire 3 unskilled people.
I don't know whether they have a different dataset of books, but the search engine is definitely vastly superior. Not only in looks, but most importantly in terms of results.
This literally means "how much do you earn as an airport in Germany". The answer is 2194€. Made me laugh.