At night, in poor visibility, when the top half of the destination platform is obscured by low cloud the reflection of the rig lights can give the false impression that you are looking at the whole rig. The sea does not have to be flat calm for this to happen, obviously, the lower the sea state the more enhanced the illusion will be.
I've seen these 'Rig Reflections' myself, but have never seen the North Sea calm enough to see a mirror image of the stars, maybe I wasn't flying that night.
I'm not going to theorise if this was a contributory factor in this incident, I'll wait for the AAIB report for that. Like most of us I have my own views on the subject but don't wish to discuss them here just with my colleagues in the crew room at work.