Odd number of matches

I research based on the number of results returned by a Bing search.
I recently noticed that if I use the same term twice in the query, I get more results than searching for the term once.
When looking for "car" bing returns 79.6 million, but when I search for "car auto", I get 2,870,000,000.

Can someone explain this behavior?

thank you in advance :)