Variants of a mold are catalogued separately, so each one has its own sets and minifigs. Check these too if something you expect is missing here.

• 3087c Window 1 x 1 x 1 Classic with Short Sill, No Glass Alternate of this part
• 3087ac01 Window 1 x 1 x 1 Classic with Solid Stud [Complete] this part is a mold of it