The cabling molecular structure continually changes and the connector interface slowly wears away. Test instrumentation specifications are critical. Some instrument manufacturers have very stringent and conservative specification procedures. Others do not. Furthermore, the language used to describe specifications differs from manufacturer to manufacturer.
Specifications can be maximum worst case, typical, or sometimes non-existent.
Specifications may state absolute accuracy or just repeatability. Overall, though, uncertainty in test instrumentation has to be significantly less than uncertainty in the phone. You should rigorously factory test your RF instrumentation to validate and guarantee its accuracy and repeatability. A false-failure phone is one that subsequently passes on an independent test system and therefore produces no fault diagnosis. Handling false-failure phones takes extra resources and equipment.