What is so difficult about all this????
To answer the original question, in part anyway, the basis for the 'one below normal category' is probably Annex 14 paragraph 9.2.3.
In response to some of the other comments......
The following are UK-orientated but ICAO SARPs contain much the same.
A licensed/certified aerodrome is responsible for providing a certain level of RFFS appropriate to the aircraft that use the place. They are also responsible for NOTAMing/notifying any temporary changes to the availability of the published category.
ATC is responsible for passing aerodrome information to aircraft using the place. A change in RFFS category would count as aerodrome information.
A pilot is responsible for ensuring that he/she complies with the relevant legislation and rules that apply in any set of circumstances. For public transport operations the Ops Manual will be the primary rule set. ANO Article 42 (c) (ii) probably sets the objective that results in entries in the Ops Manual regarding RFFS availability. Non public transport operations are covered by other arrangements. Insurance cover requirements may well set the minimum standard required.
There are lots of misconceptions and misunderstandings about RFFS cover - some of which seem to have emerged here. One I particularly like is that a reduction in fire cover that will last less than 2 hours doesn't have to be mentioned to anyone by the aerodrome. Another is that a reduction of one category doesn't need to be mentioned if the normal level is required by less than 700 movements over a three month period.
P.S. Where are all the pilots that usually come out of the woodwork to say that they're ultimately responsible for anything and everything?