Q. why does the number of results change when you put quotation marks around your search terms?

The number of results changes because when you use "quotaion marks" you are changing the search terms from individual search terms to a "phrase".

For example, if you were to search for the Great depression, you would get more than 19,000 results, but if you were to search for the "Great depression" you will get 6,300 results, because you are telling the computer to place those two words together in that order for the search.

You can find out more about phrase searching by watching this short video:

https://www.youtube.com/watch?v=Ljs3hRJWtHQ