The low weight is due to the stiffeneing board, it is just corrugated plastic as used in election and for sale signs. It is a great material for the job, very strong, tough, light and rivet-able. Most stiffening material used in panniers is to complex, and heavy and often less suitable.
The only problem with my 10yr old Super-C is that the corners of the board have worn into the material. I cut a small radius around the corners of the board to prevent any further damage and would do this to any new bags.