I did a gas rating on a boiler in a flat the gas meter supplying the boiler was in turn one of three being supplied by a main meter.
The other two meters were isolated, I took a reading from secondary meter then again out of curiosity from main meter both had revolving dial, end result. 2Kw difference
There is a Gas Safe TB about it.
As Phill mentioned in absence of any manufacture stated variance you are allowed 5% over and 10% under the badge stated figure!