A simple explanation.
Picture an airfoil section with the leading edge to the left, trailing edge to the right, and inclined at a given angle from the horizontal.
Got it?
OK, now picture relative wind coming horizontally from the left to meet the leading edge. Of course, the angle of inclination that you have pictured in your mind represents the angle of attack.
Still with me?
OK, now imagine the air flowing over the airfoil and departing the trailing edge. It is deflected downward, right? So the airflow is now no longer horizontal, but at an angle downward from the horizontal, right?
Draw all this out.
Got the picture?
OK, now here we go: If you make a vector diagram of the relative wind meeting the leading edge and the airflow departing the trailing edge...that is, join the tails of the two vectors you've pictured, and then picture the resultant...you will see that the resultant is angled down from the horizontal to a degree.
That angling down of the vector resultant represents a reduction in the angle of attack and therefore a reduction in lift.
This is exactly the situation in OGE hover.
Think this over.
Now let's look at what happens in IGE hover.
Start again with the same picture in your mind. Now, as the air moved away from the trailing edge, it's downward deflection becomes less because of the proximity of the ground...that is, the solid ground is a boundary that prevents the pressure field from bending downward so much.
Again, make a vector diagram: Join the tail of the relative wind with the tail of the less-downwardly-deflected air leaving the trailing edge in this IGE situation.
You should be able to see that less downward deflection IGE means an increased angle of attack, even though the blade angle from the horizontal has not changed.
...and therefore, more lift is produced IGE, meaning that less power is required to hover IGE, a fact which is well known.
You need to draw this out on paper until you see it. It would be easier if I could do it for you, unfortunately this explanation is the best I can do.