The amount of portions in a box of cereals, as claimed by the manufacturer: 12

The amount of portions I eat before the box is empty: 3

@fribbledom Yup. And they keep making the boxes smaller so they can charge more without the price of cereal looking as ridiculous as it really is.
@Wayves
Actually they don't make them smaller, just reduce the content.
Try opening a new one, then shale it gently to see what the fill level is. It's common to jave the upper 1/3 completely empty.
@fribbledom