True enough, 500 was the original life recommended but IIRC the modified duration was 250.
I think this has been done to play safe, and probably to try and make sure no gun gets run on too long, allowing it to get beat up by excessive slide travel in recoil.
Many of us have discussed - and I feel - that when replacing a spring with new, a slight ''set'' can be achieved by leaving slide open (put empty case in between barrel and breech face) - and also some manual cycling. This can I think take the 'edge'' off a new spring so full slide travel is good from get go when shooting.
Getting a spring excatly ''right'' in such a small gun has to have been difficult and necessarily incorporate some compromize - thus on tight side when new but allowing also for some shortening after a period of use. I do think the halving of spring life is a ''play-it-safe'' approach.