Not to sound stupid but what is PWM an is it good or bad ?
Pulse Width Modulation
Instead of a stead output of light, it is flashing very quickly and gives an apparent increase brightness for a particular power consumption when compared to a steady output (due to the persistence of vision - your eyes cannot detect very fast changes). Often the output level is controlled using PWM instead of controlling the current to the LED.
I'm sure there are a load of threads explaining in more detail, but that is it in a nutshell.
Is it good or bad?
Well if the frequency is too low it can be very annoying and visible in your peripheral vision. - Bad
It should give apparently higher output for a longer time (as it saves power by flashing the output instead of maintaining a constant output). - Good
It can give unwanted strobing affects as described with the rain water or shower. - Bad
It is neither good nor bad, but should be considered a feature which you may or may not want.
Bear in mind that all old style TVs (CRT type) flash and the effect of persistence of vision makes the final picture appear steady. These TVs typically showed 25-30 frames per second, so flashed at this frequency 25-30Hz. Even new flat panels LCD / Plasma etc flash although the frequency is usually around 100Hz. So you have experienced a similar effect to PWM