SSMChan: That's the Devs/Publisher rather than GoG. They have done the same thing on Steam too and the Discussions there are full of unhappy people with the same complaint as you.

It's a shame since the game does look nice and isn't too expensive (even without discount), but having a more expensive pre-order does leave a bad taste in the mouth even if its' only a small difference. I am not sure if I will get it though, since I played the demo and didn't enjoy the combat. I just liked the design and the levels (but they were a bit small in the demo).