When you add up all the visits from the individual projects and compare it with the number that is shown as the summary number for all visits to your site, there is a difference. We should expect to see this difference, here is why:
Let us use the analogy of a Hotel. Lets say your site is a Hotel and your projects are the rooms. You might have had 50 people come through the Hotel door to stay with you (that is the summary number). Let's say that 30 of them stayed in the ProjectA room and then 20 people stayed in the ProjectB room. But then 10 people from the ProjectA room also went into the ProjectB room (for a party! ).
So your Hotel (the site) still only had only 50 guests in total, but the ProjectA rooms had 30 guests enter and the ProjectB room also had a total of 30 guests (20 who stayed there plus 10 for the party). If you sum up every guest on a room by room basis, it looks like you had 60 guests in total, when actually just 50 entered through your front door.