In many cases different vegetables, grown in different soils, of different ages and picked at different times are analyzed. Some samples are analyzed after cooking and some are analyzed raw. There may also be variations in the portions of the plant that are used, such as the proportion of stems or leaves. All of these factors may affect the values given for calcium content of certain vegetables. Foods lose water as they cook, which causes them to become more compact, cup for cup.
Some foods shrink more than others.
For instance, spinach leaves have the ability to collapse more with cooking than do broccoli florets and stems, so cooked spinach contains proportionately more calcium compared to its raw state than does broccoli. The degree to which a food compacts with cooking also depends upon the length of time the food is cooked. When I cook broccoli, I steam it lightly, rather than cooking it down to mush. Consequently, broccoli cooked at my house may not have quite as much calcium as the same-sized serving of broccoli cooked at someone else's house where a longer cooking time has resulted in a more compact vegetable. The best way to cook most vegetables is any method that limits the amount of cooking time as well as the amount of cooking water used.