Because it's an example? You could be laying on the ground and be run over the by the tires. You didn't get knocked down, but you still got run over. That's how language works, man. I'm sorry that it's difficult for you to understand, but it literally says:
"to collide with, knock down, and often drive over <ran over a dog>"
Definition of often:
Often - Definition and More from the Free Merriam-Webster Dictionary
: many times : on many occasions
Many times or on many occasions. It can be either. So what makes more sense?
1) Dogs get run over many times
2) Dogs get run over on many occasions
I can only lead the horse to the water.