I think it is very hard to make correlations of different barrel length velocities in different rifles, or even when switching barrels on the same rifle. Different barrels will shoot different velocities, even at the same length.
A tight bore will get you more velocity than a loose bore.
Groove numbers, design, twist rate, shape, cut will net different velocities.
Factory ammo can vary a lot by each shot, each box, each lot.
Powder selection can make a big difference in velocities vs. barrel length. Faster burning powders are more efficient in shorter barrels. So they see smaller gains in longer barrels or losses in shorter.
Cartridge case capacities can make a difference in barrel length vs velocities. If you are trying to burn 100 grains of Retumbo in a 20" barrel, expect a huge fireball at the muzzle, because I would bet a lot of that powder won't be burned by the time the bullet leaves the barrel. But a .221 Fireball in a 30" barrel with 20gr of Benchmark powder has probably all burned up by 16" of barrel length. Nothing left to keep pushing velocities for the next 14".
Bullet weights can have an effect on velocities in different barrel lengths.
So many variables go into the equation, it is hard to have the same results every time. Only your rifle, barrel, cartridge, and load will tell you what YOUR rifle will do.
To me, there are a few reasons to want to shorten a barrel from 26" to 20". Lighter carry weight, stiffer barrel, more handy in the thick woods.
You can probably change your loads, even with the same bullet, and lessen your drop in velocity when cutting off 6" of barrel length. Powder, primer, seating depth.
As for physics, I have no clue how cutting off 6" of barrel length, with all other factors staying unchanged, will not effect velocity. But it does happen. I would place my money on powder burn rate.