Because, the higher viscosity puts a drag on the transfer of the ink. This is why it is so important to check and properly adjust the viscosity of all the inks before calibrating the printer. The calibration procedure sets up the printer to transfer certain amounts of ink for each of the densities in the image file. Once the calibration is done, it assumes that the viscosity of the ink does not change. If the image is printed right after calibration, the assumption is valid.
Correction calls for adding thinner to the ink and then recalibration of the printer.
In large printers with carriage tanks separate from the main ink supply, an unexpected complication may arise. When the ink in the main ink supply is thinned, the ink in the carriage tank remains at the higher viscosity. This can cause a sudden increase in density . To minimize this problem, the thinned ink from the main ink supply should be pumped into the carriage tank. This is only necessary when making a significant change in viscosity.