Michael makes some useful points about the gloss optimiser cartridge in the R1800 being used to balance out low ink areas lack of gloss, and prevent bronzing when viewing a print. However, it seems that the Light light black (LLK) ink in 4800 K3 inkset, is not merely there to balance out the metamarism and improve grey scale, but also has an unusually high resin gloss content, (higher than in the other inks) so that it acts not only as a useful member of the inkset, but also as a gloss optimiser.
When recently trying to mount some K3 prints via the Diasec liquid silicone on Plexi method, the LLK ink proved troublesome - causing the silicone to bubble, so it fails to adhere right and the mounting fails. This is almost certainly due to a higher gloss content in this ink. The other inks seem to cause no such problems. It's a clever solution, with the one caveat that it affects some face mounting choices.
It seems Epson are able to alter the gloss factor of their inks at will, and have increased it in the lighter inks (LM, LC and especially LLK) to balance out any gloss differential issues.