I also noticed that. I tend to buy the heirloom perennial artichokes which have a shorter season than the continuously planted annual varieties. Also, saw asparagus which even near the bay area I don't think is in season, most of it seems to be from Mexico.
I ran some queries and it looks like there are only about 4 weeks a year when asparagus isn't being commercially harvested somewhere in California - the first couple of weeks in December and the last couple of weeks in May (approximately)
Distribution is another matter entirely of course!