you've done the sums wrong (and as Remoak nicely showed, it is easy to do.)
Odd that, as you have already conceded that there is absolutely nothing wrong with my math.
I was going to let this go, but if you must persist...
Using the scenario already discussed:
3150' is your height
above the ground with QNH set. You know that at 13D you should be 3900' feet
above msl (13 x 3(00)). The difference should equal the elevation.
So why bother with the 3150'?
Mainly because, if you get in the habit of always knowing how far from the ground you are, you will enhance your SA, and later in the approach, you have a point of reference when the radalt starts working at 2500' (depending on terrain of course). That would have been rather helpful to the ANZ crew, I suspect.
Knowing how far above the ground you are is a useful thing. In all the years I spent going in to places like Innsbruck, Calvi, and other similar places, it was the thing I was most interested in (as others who have done those approaches will no doubt understand).
You may not like the check, or understand it, but the math was always correct. It's just a different approach to what you might do, but no less valid.
Wasn't my idea, BTW, taught to me by a crusty old BA captain who reckoned it saved his bacon a few times when banging around in Comets.