Each printer works a bit differently. For photo printers, there are often two blacks; one for matte papers and one for papers with more gloss. Some printers (like yours?) run a different black cartridge for photo / illustration prints than they do for document prints. I seem to remember looking at one Canon printer that added a gloss finish layer on prints that were printed on a glossier paper; something that is a bit similar that my Canon dye sublimation printer did where it baked on a clear glossy top coat over the four ink colours.