Google puts a time stamp whenever any user lands on a particular page then it compares the time stamp of one page with the other. E.g. Any user enters the site on Page A and moves to Page B then lands to Page C and finally leaves the site. In this case, a time stamp is put on Page A suppose its 10:10, when the user moves to Page B, another time stamp is placed on Page B, suppose its 10:20 and then when the user lands on Page C, another time stamp is placed, lets say 10:30. Now to calculate time on Page A, Google will subtract the time stamp on Page B with that of A. In this case, 10:20-10:10 which will come to 10 minutes for Page A.