LEDs have none of the thermal inertia an incandescent filament has, so their output can follow current changes in nanoseconds — any ripple in the driver's output shows up directly as flicker. Cheap drivers don't fully smooth the ripple from AC mains, which crosses zero voltage twice every cycle. Percent Flicker is a common spec but it ignores frequency entirely — a light pulsing 100% at a highly visible 5Hz scores identical to one pulsing 100% at an imperceptible 30,000Hz. Pst LM and SVM are better because they weight results against how sensitive the eye-brain system actually is, from 0.3Hz to 80Hz over a 10-minute window.