i mean that is how it works.
if he marked everyone else a tiny bit harsher (so it wasn't too noticeable), knowing that he would never mark his son, the raw and aligned marks for all students he marked would be slightly lower than otherwise.
Hence, the scaling for the subject will be slightly higher, hence correcting everyone else marks to an extent, but his son would profit from it.
I know the difference is so minuscule, but the dad could make a difference if he wanted to. (i.e. "hmmm im not sure weather to give this random kid 4/7 or 5/7. lets give him 4/7 as he's my son's competition")