Cart 1 of mass \(m\) and Cart 2 of mass \(3m\) are held at rest on a level, frictionless horizontal surface with an ideal compressed spring of negligible mass positioned between them. When the carts are released simultaneously, Cart 1 accelerates to the left with an initial acceleration magnitude three times that of Cart 2 accelerating to the right. Despite this difference in acceleration magnitudes, the total linear momentum of the two-cart system remains constant at zero throughout the expansion of the spring. Which of the following statements correctly explains this observation?