[fitsbits] Plate Center on POSS2 Plates

Hi,
I've just retrieved a fits file from
http://archive.stsci.edu/cgi-bin/dss_form
and would like to know the coordinates which the center pixel of this
image corresponds to.
I notice that there are comments "Plate center RA" and "plate
center DEC" next to keywords PLTRAH,PLTRAM,PLTRAS etc. but for the most
part, these seem not to correspond to the center of the actual fits image
since they are ~ few minutes different from
the epoch 2000 coordinates and the star of interest lies fairly close to
the center (~ tens of pixels from center). Can someone explain this to me?
Thanks,
KL