Manual exposure variations - why?

Just look at the shadow off the back of the car :

In the first shot you're close to full straight sunlight (hard shadow).

In the second shot you're in rather overcast conditions (softer shadow), probably 1.5 to 2 stops down from the first shot.

You also get different reactions in the background.

You chose the right option (go full manual) but forgot that God is not providing constant light conditions from lap to lap.

FWIW, good shots, good panning, love the subject.