Why is this 900FP Duvet classed as ultra-light?

The duvet weighs 30% less than other lightweight pure down duvets, and up to 50% less than synthetic duvets or duvets containing feathers.

This is because of two factors:

1. The extremely high quality of the down which results in less being needed to create the TOG.
2. The ultra-fine batiste cotton casing. Very lightweight but also strong enough to last.