Mass flow
The bulk movement of water and dissolved nutrients from substrate to leaf, pulled along by transpiration. Calcium reaches the canopy almost entirely this way.
Water evaporating from a leaf puts the water column below it under tension, and that tension pulls more water up the xylem. Whatever is dissolved in that water rides along. The process is passive: the plant does not spend energy moving these nutrients, it moves them by losing water.
Mobile nutrients get a second chance. Once delivered, they can be redistributed through the phloem to wherever the plant needs them, which lets a plant cover a shortfall by pulling from older tissue.
Calcium cannot. It has almost no phloem mobility, so every calcium ion in a leaf arrived by mass flow and stays where it landed. A leaf that transpires little receives little calcium, no matter how much calcium is in the feed.
This is why anything that reduces transpiration becomes a calcium question. CO2 enrichment cuts transpiration by roughly a quarter, so delivery falls even with feed EC unchanged, and newly expanding tissue, which transpires least, is where a shortfall shows first.