I think the problem is the missing life calculation. Here is a data sheet that specs pulses down to 5%. About 4X the current, but for 1/20th the time.
http://www.farnell.com/datasheets/1498098.pdf
I don't see any 20 ma LEDs with current at .4 amps at 5% duty cycle.
http://www.farnell.com/datasheets/1498098.pdf
I don't see any 20 ma LEDs with current at .4 amps at 5% duty cycle.