Printers have two types of resolution - Vertical reolution and Horizontal resolution; so in those descriptions which have the two figures e.g. 4800 x 1200 , the vertical resolution is given first (4800) and horizontal second (1200). For better printouts it is vertical resolution that is more important.
I suspect that when only one figure is given e.g. 4800, then either the horizontal and vertical reolution are identical (4800 x 4800) or they only feel it necessary to supply the vertical (better) resolution.
In the case of them displaying both the same e.g. 1200 x 1200 then either they're just being accurate or they also make printers that have non-matched resolution e.g. 2400 x 1200