Have you been driving down the road and the car in the lane next to you starts moving into your lane and then uses the signal but only after they start to move into your lane?
Have you been behind someone and you turn on your signal to move into the left-turn only lane and the person in front of you moves into that lane and then turns on their signal?
Have you been behind someone while sitting in the turn lane, be it a left-turn only or a right-turn only, and then the person turns on their signal?
I was trained how to drive in California and we never turned on our signal after moving into the next lane. We were taught that the signal on a car was to warn the drivers in back and in front of you what you are going to do, not what you have already done.