The minutes listed per player is rounded, so just adding them won't work. For example, if you had two players that played 0.2 minutes and one who played 0.6 minutes, it would list 0, 0, and 1 minutes, respectively. If you had two players play 0.5 minutes and a third play 0, it would list 1, 1, 0, respectively. So, the sum can be different even though the actual total is the same.
I assure you, you didn't have 4 players on at one point or anything :-)