I think I've found the reason for the 3 gram difference in CO2 emissions from an 04 and earlier model compared to an 05 model - it appears to be dependent on whether its a 5 seat or 7 seat model.
I've attached the following files which back this up.
04 D5.pdf
05 D5.pdf
XC90_Feb_2003.pdf
Volvo XC90 Brochure 2005.pdf
The reason I was confused over the tax banding was the price has gone up in April from £315 to £325 for a band K vehicle, and the advert was still showing the 2018 figures.