I would image part of the reason board weight is measured as purely the board weight is that the Cobra factory (which manufactures them) aren't going to include any variability in the weight of accessories in the evaluation of their board. They manufactured the board and only the board, so they outline the spec for only the board.
On the bright side, +- 6% on only the board weight means less variability (think of those extra +- 6% of the straps, pads etc. that doesn't count as board weight!
That being said, Cobra International is rated as a ISO 9002 compliant company. ISO 9xxx is an international quality standard. For a company to be ISO certified, they have to pass semi-regular audits on their quality control process etc.
I'm not sure how rigourously it is enforced, as I have worked in plants that view their ISO certification with varying attitudes. In most cases, I think it would be fair to say that they're shipping out a quality product. Then again, feedback from their customers (complaint / non-conformance report) usually helps