Answers:
Different poke about engines use different methods of penetrating the Internet. Some turn out engines use complex algorithms to look for a quantity of different variant, such as keywords (density of phrase search for), links to a site (popularity), or META tag, among others. Because different prod engines use different methods of congregation background, they come to different conclusions nearly what you be inquiring for.
To put it another style: picture I give you and a friend respectively a three-layer cake. The top layer is chocolate, the middle layer is washed out, and the bottom level is white. Your friend starts drinking the cake from the bottom up, one deposit at a time (however ridiculous that might look). You start drinking the cake from the top vein down. After you're finished, I ask you to describe the cake. Because you respectively ate the cake differently, you come out near a different perspective on what the cake be resembling.
Money compensated for advertisement. Depends on who you ballyhoo near as to what go through engine you are on.
b'coz of different databases in the query engine