ICAO Cat C vs. D
This post may have been answered.....if not,
I am an ICAO pilot and I have gone to the ICAO source document 8168 to provide an answer, I apologies to FAA/JAA operators but I believe the answer is the same.
For certification, the speed dependant table is based on a calculated Vat (Vref) at MAUW and is the higher of 1.3 Vso or 1.23 Vslg (with the landing configuration defined by the manufacturer.)
ICAO then specify the following:
An aircraft must fit into and be operated in accordance with the
requirements of only one category. An aircraft:
(a) may not reduce category because of reduced operating weight; but
(b) must increase category when actual handling speeds are in excess
of those for the category.
Your aircraft will fit into Cat C using the pure calculated certification but Boeing can provide higher Vref speeds due to handling considerations unrelated to Vso or Vslg.
This is allowed (and expected) hence the provision in para (b) if actual Vref puts you in a higher Cat.
If you are wondering about para (a), an operator may specify a permanent reduction in the MAUW for their fleet, allowing their fleet to be permanently certified to use a lower Cat. Para (a) is eluding to this rule, as the change must be permanent and certified.