Amy Joy Lanou, PhD, Senior Nutrition Scientist at the Physician's Committee for Responsible Medicine explained in her March 2005 paper "Calcium, Dairy Products, and Bone Health in Children and Young Adults: A Reevaluation of the Evidence," in Pediatrics:

“Dairy products contain nutrients, including protein, sodium, and, in some cases, supplemental vitamin D, all of which influence calcium balance and bone mineralization and alter or negate the effect of dairy calcium in the body’s mineral economy. Animal protein and sodium, in particular, tend to increase calcium excretion…

We found no evidence to support the notion that milk is a preferred source of calcium… Although milk and other dairy products are reliable sources of calcium, many factors affect the availability and retention of the calcium from these products. For example, the calcium in dairy products is not as well absorbed as that in many dark green leafy vegetables but has an absorption fraction similar to that of calcium supplements, calcium-enriched beverages, calcium-set tofu, sweet potatoes, and beans. Dairy products… clearly increase the urinary excretion of calcium as a result of their increased sodium, sulfur-containing amino acid, and phosphorus content.”

March 2005