According to my Senior Engineer when I was a mere Junior, regarding comparison of the AVO 7 and the AVO 8:
The AVO 8 is more sensitive because its movement's full scale deflection (FSD) is 50 μA (microamps). It consequently needs to 'rob' only a little current from the circuit under test. This will minimise the difference between the indicated and true voltages when the circuit under test presents a high source resistance. This low value of FSD requires high Ohmic values for the AVO 8 multiplier resistors and other internal circuitry. These high Ohmic values require, for reasons of economical manufacture, the use of metal film or hi-stab carbon resistors which are less intrinsically stable than wirewound resistors.
The AVO 7, on the other hand, is less sensitive because its movement has an FSD of (if I remember correctly) 1 mA (milliamp). This means that the AVO 7 needs to 'rob' more current from the circuit under test leading to more disturbance of the circuit and a greater difference between true and indicated values, the more so if the circuit under test presents a high source resistance. However, the higher FSD of the AVO 7 movement leads to lower Ohmic values for its multiplier resistors and other internal circuitry. These lower Ohmic values are consistent with the economic use of wirewound resistors. These are intrinsically more stable than metal film or hi-stab carbon resistors so the AVO 7 is more precise than the AVO 8.