ansi 9standards0 code issues - ansi 16.1, 16.2, 16.5 sta

huangyhg

ansi ansi  16.1, 16.2, 16.5 sta
hello,
i work for a general contractor.  i'm detailing service connections and meter turnout vaults for a water supply pipeline project.  the pipeline itself is 60" steel and rated for 300 psi.  there are 12" & 24" service connections which have ansi  16.5 class 300 slip-on flanges.  at each of these service connections there is a vault which contains a cast iron butterfly valve (ansi  16.1 class 250) which is bolted to the class 300 steel flange.  following that connection sequence is a flange coupling adapter (fca) for connecting dip and a flow meter.
my question is about the fca which my supplier has indicated meets awwa c207-01 ansi  16.2 specs.  my research indicates that the fca steel-ring flange has the same diameter & drilling as the valve body but, the steel-ring flange is thinner (1.63") than the flange on the valve body (2") and thus, affects the length of the bolts for connecting the two.
can someone clarify why the fca shouldn't have a 2" thick steel-ring flange that is ansi  16.5 class 300 for the 12" service connection instead of a 1.63" thick flange that is ansi  16.2 class 250?