V-Ring seal VS18-FPM

How to read the reference of a v-ring seal?
Example: 
VA20-NBR 
Where 
 VA = gasket shape 
20 = shaft diameter on which gasket will be mounted (approximate) 
NBR = material 
How to read the suffixes of a v-ring? (type of rubber)
FOR OILS AND HYDROCARBONS NBR = Nitrile = Butadiene rubber - Acrylonitrile
        from -30°C to +110°C FPM = Viton = Fluorinated Rubber
        from -20°C to +200°C

My seal does not make the dimensions indicated on the site, why?
The dimensions indicated for the V-RING type seal are those of the joint mounted. 
They must therefore correspond to what is indicated on the technical sheet when the seal is mounted on its axis.