I set a magnification of 100.5% and it does a magnification that I would say is 105%.
As communicated per email; this is probably due to the X and Y dimensions of the photon mono 4k were wrong.
They are wrong in photonworkshop and it seems to be using 35micron as a resolution. Using that resoluation you will end up with a larger build platform for 3840 x 2400 pixels than is publicly given as size. That led to the difference.