Good Q and also, what form of calcium was used for the study. Many common forms used in supplements is anot al that easily assimilated, and thats what counts, by the body. Cal carbonate is most common - and is very hard for the body to assimilate. D helps with its use by the body, but still only a small % is actually going into the bloodstream.
Calcium citrate is probably the best form for humans. It should be accompanied by Vit D, and some Zinc for best use by the human system...;-)!
I think I've also heard that to metabolize calcium into bone, you also need magnesium and phosphorous, thats why most of the supplements have all three.