Why might a butterscotch cream pie, which is high in sugar, have half the sugar added after the cornstarch-milk-egg mixture is already cooked? If all the sugar were added to the butterscotch cream pie...

Why might a butterscotch cream pie, which is high in sugar, have half the sugar added after the cornstarch-milk-egg mixture is already cooked? If all the sugar were added to the butterscotch cream pie before the mixture is cooked, what might happen to the texture, appearance, and mouthfeel of the pie?