Most people play shafts that are too and too heavy stiff for them. These shafts also vary in flex. That is why most people hit the ball to the right and are inconsistent. They subconsciously compensate with their swing for the differences. Trying too hard to square the clubhead at impact can lead to numerous swing faults as well as fatigue and back problems.
Manufacturers build clubs by the thousands.
They buy case lots of shafts that fall within general parameters of weight and stiffness. Therefore, every shaft varies in flex. Grips, lie and loft are standardized to fit the general population. What we have now are clubs built for an average population that fit nobody specifically. Frequency matching is measuring the cycles a shaft flexes over a set period of time.