You're quite correct Sailor.
I travel myself quite alot, and have got used to interpreting advertised rates (and sometimes between the lines for supplements).
Basically the 18 Euro price would be based on two people sharing a room, and taking breakfast (15 Euro each without breakfast).
So effectively you were charged an under occupancy supplement (For the "priviledge" of having the room to yourself).
I realise this may not always seem fair, but in many cases there's not much you can do about it.
I do believe that the winter rates advertised by this hotel still provide good value, even for us singles.