I believe that with the longer barrel like you have that there is more dwell time in the barrel so the 22 Long Rifle has a bit more time to bump up thus increasing.
Pressure peaks as the bullet starts to move and is typically decreasing from there on out. If the bullet is gonna obdurate to fit the bore, it'll do it then, not later on.
However, the softer lead theory is sound - I'm not sure there are many .22 rounds that are too hard in any case.
Dedicated .22WMR barrels are, I think, only 0.001 larger in diameter (0.0005 to each side) than the average .22LR, which already vary widely.
Paco Kelly's Accurizer (sp?) tool lets you size your .22 bullets from .222-.225, I think, depending on the model. Gunblast had an article awhile back with very good results in a single-six, likely helping the .22LR's fill the .224 chamber throats & bore. I haven't noticed an improvement in my CZ rifle, but it has a tight bore (measured w/ several re-crowning pilots) and shoots most ammo pretty well.
Ultimately I suspect that most often the accuracy noted from long-barreled pistols is due to the long sight radius, nothing more. Sight alignment is more precise with a longer barrel, and that DOES make a noticeable difference in my hands, both with rifles and handguns.