WBS
I'm confused by your table of numbers. If you're attempting to work out the change in level corresponding to a change in pressure as a function of level in the ISA, then they're incorrect.
The variation is much higher -- some approximate numbers are shown in the
pressure gradient section of the reference you quoted. Even those are broad averages.
I previously used 100 ft/mbar in the mid-300s, which comes from the knowledge that the 250 mbar level is conventionally FL340, and the 200 mbar level is conventionally FL390, so that's an average of 100 ft/mbar.
Maybe you're using the average gradient between sea level and the level indicated?
If that's what the numbers are meant to be, then I can only reiterate that it's not relevant, for the reasons that I've tried to set out.
If I try to put it another way, you need to realise that changing the altimeter setting by 1 mbar is
not the same as moving the aircraft up or down by one mbar of pressure in the atmosphere. When you change the altimeter setting, you shift the
reference level, at which the altimeter would read zero.
So in your example, your aircraft at 20,000 ft on the RPS of 1033 is 20,000 ft above the 1033 mbar level. Your aircraft at FL200 is 20,000 ft above the 1013 mbar level. The vertical distance between them is therefore the level difference between the 1033 mbar level and the 1013 mbar level, and that's about 20 mbar x 30 ft/mbar = 600 ft because the 1033 mbar level and the 1013 mbar level are both close to sea level.
If you were to measure the actual pressure outside the FL200 aircraft, it would be about 435 mbar. If you were to measure the actual pressure outside the 20,000 ft on 1033 aircraft, it would be about 446 mbar. There would only be 11 mbar difference in pressure between them, even though their altimeter settings are 20 mbar apart.