During change over, from PK or MK ink, the Epson flushes some ink. This is 2 to 4 ml per change. It is prudent, in fact frugal, to batch your glossy work and your matt work, instead of intermixing the items.
Fine art papers use that matte ink, while pearl and gloss use that photo black ink. This is controlled by the media selection you make in the epson driver.