The SR is a "Corners Per Incident" (or CPI) count basically, where for Rookie you look at less corners than for D-Class, which is less than C-Class etc. So A-Class drivers have both a harsher modifier to get from CPI to the SR number but also have to maintain that level over more driving.
So if you do 10 corners, and get 4 incidents then that both adds 4 incidents to your total but also knocks 10 corners out of the 'back' of the queue. Now if those 10 corners already had 4 incidents you'd see limited/no change to SR, if they had 8 incidents then your SR would improve even though you got incidents or if they had 0 incidents then you're SR would go down. simples ¬_¬