Well, maybe because they're longer/heavier and there's a bit more pressure built up, that allows that powder to burn a bit longer, but there should be no inherent reason why a shorter barrel should shoot a particular bullet better, than that same barrel in longer form.
I'm just throwing stuff out. My 338LM sports a 1:12" 27.125" barrel, so I'm kind of stuck with the 250s, but those 250s, if punched out up to ~2950 fps, will shoot flatter than the 300s (at typiical 300gr velocities) out to ~1500 yds.
The 300s really shine in longer barrels, single feed and at distances approaching that mile marker.
Unfortunately, with a 20" tube, you're probably not trying to get to one mile, so why bother with the 300s, I guess is my thought on the matter?
Different strokes and all.
Good luck, Chris