I wonder if its down to the fact that when a team gains promotion and plays in a higher league, the QUALITY difference between teams dosen't have the same effect, because players quality is much lower. What I mean is:
In, say level 3 or 4, if your team is 8-15 quality better than others, that is quite significant gap, and therefore you can score goals more easily. However, an 8-15 point quality gap in say level 9, is not such a vast difference, so goals are less easy to get.
This is my only my personal theory.