Disinformation. The driver, SS Agent Greer, turned TWICE to look at JFK. When he saw that JFK had not yet been mortally wounded he slowed the limo nearly to a dead stop. It was THEN that the fatal shot occurred.
It was all planned. There was a section of the curb painted white. When Greer reached that spot his direction was to check on JFK and slow to stop if not yet mortally wounded. This is clearly seen in the must watch documentary “Everything is a Rich Man’s Trick”.