Here's why:

* Center of mass depends on both mass and distance: The center of mass of a binary system is determined by the masses of both stars and their relative distances from each other.

* Example: Imagine a system where one star is twice as massive as the other. If the more massive star is twice as close to the center of mass as the less massive star, they will actually be balanced around the center of mass.

In general, the more massive star will be closer to the center of mass, but this is not always the case. The specific location of the center of mass is determined by the ratio of the masses and the distance between the two stars.