Because the Sliversea ship emits less CO2 or whatever than the QM2 does? How can they tell that? I mean how can they measure the emissions of the QM2 short of putting in dry dock an running and emissons test or put some type of pipe into the ships exhaust? Neither of which have done or could do? And haven't larger ships than QM2 been there and older ships?
FWIW I work in garage in sales and our MOT tester - MOT is uk test for car safety and function including exhaust emissions americans have similar test - our tester reckons there is no way they could do it without doing either of the thing I have described?