The INS voltage meter only matches the actual output if the appropriate correction provided by the equipment firmware is selected.  The default selection is 53.5 ohm termination: 53.5 ohm (builtin-terminator)/(53.5 ohm characteristic impedance of the coaxial line + 53.5 ohm built-in terminator). 

The use of AT-810/00-00017A attenuator means that the coaxial line is terminated by the input impedance of the attenuator, therefore, the numerator and denominator should change to 50 and 53.5 + 50, respectively.  The maximum allowable setting on the INS voltage meter when using either attenuator should be decreased accordingly.