Odd number of matches

Hello,
I'm doing some research based on the number of results returned by a Bing search.
I recently noticed that when I use the same term twice in the query I get more results than a one time search.
For example, when looking for "auto", Bing returns 79,600,000, but when I search for "auto car" I get 2,870,000,000.

Can anyone explain this behavior?

Thanks in advance :)