The 1.5 Auto is now a mild hydrid and as this is something of an afterthought they have positioned the battery pack in the boot and not under the passenger seat as that space was taken by the ‘JS24 Digital Sound Package control unit’. This means that the boot floor on this model only is not able to ‘drop’ hence the reduction of quoted litres.
As most will be choosing the S&C pack for the heated seats, this also adds a subwoofer in the boot where the spare wheel would usually go so this may affect the ability to drop the boot floor in other models too. It’s a case of seeing a car with the ‘pack’ fitted or a Q3 with the mild hybrid batteries fitted but the figures are correct.