It doesn’t bother me if they signal before moving over. It’s stunning to me that people signal before turning and not before slowing down. The whole point of the turn signal is to give warning, yet some people don’t signal until they are already making the turn. Forget the people who don’t signal at all.
They slow down, because they feel like they need to. Maybe their stop is more gradual than yours. My husband doesn’t start stopping soon enough for me a lot of the time. I don’t know if he saw the red light, yellow light, stopped car, lane closure, is forgetting to turn, etc. Makes for a lot of arguments.
Edit: Also, some people see better than others. I think people only need to be corrected to 20/40 to drive. Also, people like my dad have bad depth perception so he is more cautious about estimating his distance from an object, intersection, etc.