QTR-Quadtone RIP

Hi everybody,

while trying to print at 2880dpi I've know realised that all my images 
are consistently stretched vertically by about a factor of 2, even
the initial ink pattern page in calibration mode and the 21step test 
prints. Is this a known issue or may I have done something wrong? I 
choose 2880 because the initial ink pattern page looked very smooth 
when compared to the 1440dpi printout (even if it was stretched)

regards, Jamie