Older wooden forms would shrink with time. The impact is different based on the direction. If a board is cut such that the length of the board is oriented towards the vertical growth of the tree, shrinkage is less than 1%. But if the length of the board is oriented towards the width of the tree, the shrinkage may range from 2% to 12%. If the size of the block is say 4 feet, and the boards were cut across the width of the tree, those boards would over time show quite a bit of shrinkage.
Now in a modern lumber mill, boards are cut down the log, which orients towards the vertical of the tree. But suppose you are cutting a board by hand directly from a tree stump. In that case, the boards are cut across the width of the tree, and these boards would show an average of 5% to 10% shrinkage over time. In this situation, if the boards are used for several years, the wood shrinkage of the planks used for forms would result in noticeably different block sizes.