HP 758X B series plotters require that C (as well as A2 metric) paper be loaded "length along platen," and not "width along platen," the way B and D size are loaded. The Windows driver for the HP 758X B series plotters calls for C size paper to be loaded "width along platen" (which the device will not allow). Consequently, when you choose Portrait, you get Landscape, and vice versa on C size paper.
The solution is to change the orientation of C size plots by using the HPGL command RO, or by using the Rotate command on your HP plotter's console.