This provides an alternate explanation for the correlation between broken bones and the beverage consumption. If teens who drink the beverage drink less calcium-rich beverages (ex. milk) than other teens, the disparity in calcium-rich beverages could be the primary causal factor.

This compares teenagers vs. older people. We want a comparison between teenagers who drink the caffeine-rich carbonated beverages and teenagers who don’t drink the beverages.

The author never assumed that calcium deficiency can never be caused by anything else besides caffeine consumption. The author’s theory requires only that caffeine consumption increases the likelihood that a child will have calcium deficiency and broken bones.

The correlation and the conclusion do not make any distinction between different kinds of carbonated beverages with caffeine.

So, eating more calcium leads to more excretion of calcium. This doesn’t change the fact that eating caffeine causes people to excrete a lot more calcium than they otherwise would.