Hi Karl,
Thank you very much for the question and the hint! It is correct that the six-layer boards have a thickness of approx. 1.4 mm - not 1.6 mm! Please see here.
However, when adding up the individual layers, I get 1.454 mm. I guess the communicated value in the specifications of 1,404 mm does not include the solder resist.
Best regards,
Manuel