I don’t think Californians improve that much from Woodbridge-Clovis-Mt Sac-State-Nationals. Good Californians are more consistent if anything. They do get better at running harder courses, usually the 1st hard course is a learning experience.
If someone had the best race of their life at Woodbridge, it is unlikely that they will improve significantly. If you don’t believe me, look at their track season. Best races often come early-mid season.
Weather is usually significantly hotter at Clovis than at State. That is why we see the improvement. 75-80 degree weather compared to 58-65 degree weather matters a lot. Arcadia 3200 would not be the same if it were run in 75 degree weather. 58 degrees is why Arcadia is so fast.
In California, it is all about racing hard all the time. The smarter teams will make sure they save some left for the end of the season. Each big race feels like a national championship race. As I said before, look at the track times, they are racing constantly trying to pr.