$18 on Frictional Games Store and only $16 on STEAM

[Temar]

It's not a guess. It's a fact.

Need confirmation?

We do not set the price for the discount in other stores and did not know Steam would go for 20%.

Sorry, Jens' quote does not proof your theory at all. In fact, if Steam was the publisher of the game, FrictionalGames could not add it to other direct download services nor sell it on their own homepage.

So please don't promote something as a fact if you don't know it at all. Just let someone official answer the question.