You can't have the ball if the other team has the ball. Overall time of possession depends on your offense AND your defense. If your offense holds the ball for four minutes every possession, that's extremely good. But if your defense allows the opponent to hold the ball for six minutes every possession (which is terrible), you'll lose the TOP battle, and your offense will end up with 24:00 of possession compared with your opponent's 36:00. On the other hand, if your offense holds the ball for only 3:00 per possession, and your defense limits the opponent to 2:00 per possession, you'll end up with 36:00 of possession, compared with 24:00 for your opponent. So, in the first case, you had the ball for only 24:00, and in the second, you had it for 36:00, but your offense was actually much better at maintaining possession in the first case -- the difference is that your defense was much worse.
That's why looking at TOP per possession is much more indicative of each side of the ball than just looking at the overall TOP per game.