The best explanation for why the total momentum of the soccer balls before and after the collision may not be the same is:
The system has external forces, such as friction and air resistance, acting on it.
In an ideal isolated system with no external forces, the total momentum would be conserved. However, in real-world scenarios, factors like friction with the ground and air resistance can affect the momentum of the objects, leading to discrepancies in the measured total momentum before and after the collision.