With devices that have equal resolutions or DPI in both direction, yes, it is common to not repeat the equal vertical and horizontal numbers, but when the numbers are not equal, it is indeed common to give both. For example, the Epson R1800 has a resolution of 5760x1440DPI. If you increase image detail only on one axis, resolution increases linearly, proportional to the increase on the single axis. But when the resolution on both axes change equally, the overall resolution is the square of the resolution on either axis. 19MP is a significant, visually obvious improvement over 13MP when individual pixel quality is similar in both images.