Why does the contraceptive pill "Diane 35" only contain 21 pills per pack??? - triphasil contraceptive pill

In k would now .... Ring Mae pills I really stupid, just the changes in product triphasil (South Africa) to Diane 35 (now living in France), but these pills contain 21 pills Diane as excessive triphasil 28, including the area "Red" begins much obvious. ... . As Diane works if your only 21?