I have found that with all sorts of leds.
I think turning them on and off shortens the life span!!
You might get 20k hrs but only if you run them constantly, in ideal conditions, fed from a lab power supply.
To be fair they have mostly lived longer than the warranty period. And the 240v g10's I had in my kitchen only last a few months, as they blow I have been replacing them with leds and they have all been ok for coming up a couple of years now..
Well the ones that failed are the ones I leave on all night. I leave a strip of 3 on and 2 of them have gone so far.