I've attached some images to clearly show the problem.
I've double checked the cabinet dimensions, and they are indeed 630mm across using 18mm panels so I honestly have no idea why it's not fitting.
The panel I have is "standard" WMS Widebody (5mm thick, 603mm wide, 1092mm long).
Looking at that back photo it doesn't look like 603mm would fit in the first place, so I'll just leave here confused and go and order another sheet...
