Why is the center of gravity and balancing a ruler and a book different?

The center of gravity of an object is the point where all of its weight is considered to act. Balancing a ruler and a book is different because the distribution of mass in each object affects where their center of gravity lies. The ruler is likely to have its center of gravity closer to its center due to its uniform shape, while the book may have its center of gravity farther towards the bottom because of its uneven weight distribution.