2,400mg refers to the total amount of magnesium glycinate per 4 capsule dose. 240mg refers to the elemental value of magnesium. This is the magnesium your body can utilise.
Magnesium citrate for example, has a higher elemental percentage, however it has a much lower bioavailability so is not as well absorbed. So, whilst genuine non-buffered magnesium glycinate only has around 10%, it is much better absorbed by the body and therefore better utilised.
When Magnesium Glycinate has been buffered with another form of magnesium (Mg Oxide for example), the elemental value will be higher than 10%, but Mg Oxide has a much lower bioavailability than gycinate so in this case, less is more.