Hi, SJ,
I am also a California driver, and I will give you one reason I signal late. Because in the days when I was trying to be a "nice" driver, and I would signal far in advance a whopping 99% of the time the driver in the other lane would speed up so I could not change. It got to the point where I would almost miss my off ramp.
So, not to in any way imply that you are wrong for being irritated, that's just my reason. And, let me add, I NEVER cut other people off when I change lanes, I am always very safe.
Happyout (who is usually irritated with California drivers, too)