You need to explain your question better....like who says this?

For a small block Chevy, the deck height is 10.400 inches; that is, the distance from the crank centerline to the top of the block surface where the heads bolt. This distance is equal to the sum of 1/2 of the stroke plus the rod length plus the piston height (from the piston pin to the top of the piston) plus how far the piston top is below the deck surface (a minor amount). A piston therefore is designed to fit the above equation. Also, on a piston, there is the skirt that is below the piston pin. The skirt is designed to clear the crankshaft counterweights....if a piston designed for a long rod is put on a short rod, the skirt would probably hit the crankshaft and the piston height would be so short that the piston would not go to the top of the bore..... and have low compression. Therefore, the pistons must match the rod length that the piston was designed for. Is this what you are asking about?