I've noticed all sorts of weird stuff happen when you transfer FITS files between different image processing apps. The PI guys argue, usually convincingly, that they are right and everybody else is wrong
The basic problem is that the FITS file standards aren't complete and there is freedom to interpret them in different ways.
One example is the coordinate origin of images. I think that Maxim and CCDStack assume an upper left corner origin. PI defaults to lower left corner, but you can change this in the FITS Format preferences.
A bigger problem is that the FITS standard doesn't specify the black and white points for floating point values. PI assumes a black point of 0 and a white point of 1 but other apps use different ranges. Once again, PI lets you change this if you need to.
Cheers,
Rick.