Angolul értők: miért "jön a felszínre" a vaj, és marad alatta egy szürkés, zavaros lötty?

"1 cup (2 sticks) unsalted butter, softened

6 to 8 cups confectioners’ sugar

1/2 cup milk

2 teaspoons vanilla extract

Place the butter in a large mixing bowl. Add 4 cups of the sugar and then the milk and vanilla. On the medium speed of an electric mixer, beat until smooth and creamy, about 3-5 minutes. Gradually add the remaining sugar, 1 cup at a time, beating well after each addition (about 2 minutes), until the icing is thick enough to be of good spreading consistency. "

A 4cups cukornál én megálltam, elég édesnek gondoltam.