Remember that, historically, there was that water ingress problem on the Pre facelift R230 because of the shape of the boot lid. As a result, both the designer and user would have been wary of the thing springing open, and dropping water into the boot, when the bootlid was wet.
All that said, on your 13 year old SL, the dampers may be a little tired, and not lift the lid as much as was intended. In good order, they should lift the lid enough, and easily go the rest of the way in your hand.