The fixed reference point might be evidenced differently depending on the particular flying qualities of each aircraft type. This fixed reference point seems to have been alternately referenced as stall, point of zero lift reserve, onset of mushing flight, etc. The LRI is a much more useful tool than just being a stall warning device. It gives a repeatable indication of all these performance maxima and minima independent of all the variables given above. Most of the values listed in the manuals of current production aircraft are somewhat conservative.
Manufacturers are very liability conscious, especially in the low-speed regime.
But there are some variations in performance from aircraft to aircraft, even off the same production line, and even more variation in static source position error from one airplane to the next. It certainly builds the owners confidence and proficiency as well. Sometimes it is hard to bridge the gap in understanding between engineers and pilots.