There are a few reasons why you may see the cartridges drain at a different rate-
1. There is a clog in one or more of the nozzles. A blocked nozzle will prevent 1-100% of the ink from pulling from that slot, thus throwing off the amount of ink is in that particular cartridge compared to the others. Performing a nozzle check will tell you if this is the case. Click here for info on how to do a check.
2. Using Accurip, one or more slots are deselected, telling Accurip that you do not want to pull ink from those slots, when printing Accurip to Epson.
3. Accurip is configured for Multi-Black printing, however, Accurip is not being used on 100% of prints. Some printers will assume that because halftones are not in the design, they should not be printing Accurip to Epson. To ensure that ink is pulled as evenly as possible and utilized properly, all print should be printed Accurip to Epson.
4.Only slots 7 & 11 are off. The 4900 is built to use either Matte Black or Photo Black when printing with the standard Epson ink. The user would use Photo Black for photographs or glossy surfaces, and use the Matte Black for matte substrates, such as text to paper. As this is how the printer is manufactured, Accurip has to be told which slot to be set to. You'll notice that when you to go to File > Setup or File > Edit Configuration in Accurip, at the top, you must choose the Matte Black or Photo Black via the drop down. Due to how the printer is built, there will be some back and forth switching between the Matte and Photo Black options, when swapping out sets of ink cartridges.