The relevant European safety standard for strollers, EN 1888, specifies a minimum angle of 150° between seat bottom and backrest for it to be suitable for use from birth.
The Stokke® Xplory® /Crusi™/Trailz Seat does not comply with this specification, which is why, in the EU, it is not accepted for use under the age of 6 months. In the UK, however, a different standard applies which does not include the above requirement and therefore the Stokke® Stroller Seat (for Stokke® Xplory® /Crusi™ /Trailz™) is deemed suitable for use from birth in the UK.