The phrase "detectable difference" refers to the smallest change or variation in something that can be noticed or identified. It represents the point at which a change becomes noticeable or distinguishable from the original state. Full definition
A recent experiment, for example, showed that six weeks of cognitive training could produce detectable differences in brain microstructure. (sciencemag.org)
On the other hand, in this study, looking at three «subspecies» of common chimpanzees, «we found significant genetic variation,» said Przeworski, «but there's very little detectable difference between the populations in terms of appearance or behavior.» (uchospitals.edu)
Optional adaptive dampers can be had which offer 3 modes, Normal, Sport, and Comfort, which provide detectable differences in the ride comfort, albeit not by much. (carbuzz.com)