In many cases, there is a huge distinction between "build the most accurate and precise possible piece of apparatus for measuring X" and "build the most accurate and precise piece of apparatus, within certain constraints of cost, size, power consumption, data acquisition speed, resistance to environmental effects, etc." If one is trying to design a practical device which has to meet any sort of tight constraints, and if one can afford to have some of those constraints on a test-bench setup (since, among other things, one will probably either want to build more than one of the device one will be testing, or one will want to use it in places or circumstances where one wouldn't have to use the bench-test setup.

In many cases, there is a huge distinction between "build the most accurate and precise possible piece of apparatus for measuring X" and "build the most accurate and precise piece of apparatus, within certain constraints of cost, size, power consumption, data acquisition speed, resistance to environmental effects, etc." If one is trying to design a practical device which has to meet any sort of tight constraints, and if one can afford to ease some of those constraints on a test-bench setup (since, among other things, one will probably either want to build more than one of the device one will be testing, or one will want to use it in places or circumstances where one wouldn't have to use the bench-test setup) the test-bench setup may in many cases easily be more accurate than the device being evaluated.