Inkjets work by dragging an inkhead across the substrate. The inkhead has a number of closely spaced nozzles which are "pulsed" at a high frequency (up to 50KHz) as it moves across the substrate. As you can see from the image below, the resolution along X and Y can differ because one is determined by the spacing of the nozzles and the other determined by the relative velocity of the head/substrate and the pulse frequency.
The InkRaster program supports different DPIs in X and Y. The -dpi: argument can take either one parameter or two. If only one parameter follows then both DPIx and DPIy are assumed to be equal to the parameter. If two parameters follow the -dpi: argument (separated by a comma) then the first is the DPI along X and the second the DPI along Y.
Both DPIx and DPIy = 2500
DPIx=2100 and DPIy = 2400