I’ll try again.
Ca~Citrate (weak bond, comes apart easily in water)
Ca-Carbonate (medium bond, doesn’t come apart as easily in water)
H~oxalate (what’s in your food, weak bond, comes apart easily in water)
Ca=oxalate (strong bond, relatively insoluble in water)
[Ca and H have positive charge; citrate, carbonate and oxalate have negative charge - + and - like to stick to each other, but with different strengths]
When you take Ca~Citrate, the weak bond comes apart easily and the free Ca gets absorbed from the gut into the bloodstream right away.
When you eat foods or drinks with H~oxalate, it comes apart easily, and unless there is Ca around to bind to the free oxalate, it gets absorbed into the bloodstream relatively quickly.
But if you have Ca-Carbonate in the gut at the same time, the oxalate from food can knock the carbonate off the Ca and replace it before the oxalate is absorbed into the bloodstream, to form Ca=oxalate, which is insoluble and precipitates in the gut and gets pooped out (along with some of the Ca-Carbonate that doesn’t come apart in time in our short guts).
If you only take Ca~Citrate, not as much is available to bind to the oxalate, and the part that does bind is pooped out. That’s I believe why if you’re a stoner, you should take Ca-Carbonate with your meals and not count it towards your daily Ca requirements.