Carat is weight, not size
Two diamonds can weigh the same and still differ in diameter, depth and proportions. That is why ct↔︎mm charts are useful for estimating but are no substitute for the stone’s actual measurements.
Where can the weight go?
A diamond with a small diameter for its weight may carry more of that weight in its depth, its girdle or other proportions. For CAD these differences matter, because the head and prongs are built around physical geometry.
What is needed for a center stone?
Ideally, its physical measurements. When the data points to an unusual stone, or when an especially precise fit is required, a 3D scan of the actual stone can also be used.
So why are charts still useful?
They provide a shared language and a quick sense of scale - as long as they are treated as an estimating tool and not as a promise that a given size always equals a given weight.