short of Milk in a 1 L

hi i recently purchased a 1L full cream Milk box - as i needed to bake a cake which required 4 cups of milk - to my dismay - the box only gave me 3 1/2 cups of milk - i was totally shocked - i had to make up the milk with water and make the cake - then i filled up the box and tried again filling a 250ml pyrex jug to measure and still 3 1/2 cups of milk - please advise - i googled and 1 L = 4 cups = 250ml each ?? 250mlx4 = 1000ml = 1 L .. please explain - i am a baking and always buy C***** 2 L but never measure - now im so concerned - am i being ****** of milk- without knowing -