LED's pulsed for say 200% of their 'normal' current at a 50% duty cycle appear brighter than 100% current for 100% duty cycle even though the maths says that it's all the same average current. Provided the refresh rate is sufficiently high. Your eye retains an image for ~ 1/60th second. If you go slower than this, the image fades and you can perceive the flicker. If you go faster, the decay rate is such that it can appear brighter. This may have been the reason for the choice of 60 cps AC power line.
The attack time is significantly shorter than the release (or decay) time, making the eye tend to be a peak sensing device rather than an RMS detector. The human eye is responsive to the peak value of illumination. It is assumed here that the pulses have a repitition rate greater than 24 pulses per second. Otherwise, the perceived intensity is also a function of the repetition rate. Above about 40 pulses per second this perceived increase in brightness levels out and is no longer a factor. this varies greatly between men and women, eye (and hair!) color (melanin content), age, and heritage.
See also: