Assuming that Newcastlebloke got it right in saying earlier that "The original post is correct; overtake on the right in the air, and left on the ground", surely the reasons are not difficult to see.
In the air, a potential head-on conflict is to be resolved by altering course to the right. Like ships, aircraft are require to pass port to port. Whether an aeroplane with pilot in the left seat or heli with pilot on the right. So if you overtook on the left, and the overtaking aircraft were in potential conflict with a head-on one coming the other way, and/or following a line feature which it is required to keep on the left, overtaking on the left would put the overtaker at risk of hitting the conflicting aircraft or going the wrong side of a line feature. With no lateral restriction on where to go, it is possible and safer to go right.
On the ground, if an overtaking manoeuvre is possible, it can only be where there is a fairly wide manoeuvring area. Then it is like a multilane highway in the USA or continental Europe. Normally a single aircraft would have to keep to the right, giving oncoming traffic the widest possible clearance. If you overtake (if safe to do so) you have to move to the left - just like getting into an overtaking lane on a USA/continental highway - because the one being overtaken is already as far to the right as it can reasonably be. This puts you closer to traffic coming the other way, so you have to be sure it is safe to do so - that the "road" is wide enough.
What is hard to understand about it?
Chris N.