Most likely that bumper was repaired/repainted before you bought the car...even if you bought it new.
I was in a Ford Dealership the other day. Way way back, at the back of their long and deep lot, were subcontractors painting and doing minor repairs outdoors on two new cars....painting their bumpers. It happens, it's just normal SOP in a Dealership. And an 'adjustment' may have been made to the car anywhere between the factory and it's final delivery to the Dealership as a new car...even the Dealership would not know.
Personal experience: I bought my 05 GT when it was just under two years old. It's lst owner was a Ford Factory rep...one year. It's second owner/driver was the owner of a Ford Dealership...he drove it 9 months. I bought the car at a steal price. Noted the front bumper had slightly peeling paint, on it's upper edge, where it meets the lower sharp surface of the grille. I figured the entire bumper had been repainted at some point.
And just last week I accidentally rubbed the rear bumper past a wooden fence...some paint just rubbed off the bumper...perfect, shiny, non damaged black plastic showing underneath.
So these bumpers are made of a soft plastic material, which paint does not adhere to well.