Staying with a wheel gun. Probably one large contributing factor on accuracy of .38 could be that like JumboFrank said, the .38 has had a lot of load development behind it. That is a always a big factor in accurate loads.
As far as comparison with machine rest, I don't know of anyone doing the comparison of one gun or model and .38 or .357. What is true for rifle or pistol bullets, velocities and different bullet weights make a difference. Action, barrels and a lot of other things also influence accuracy. A given bullet, say .357, 158 gr travelling at 800 fps may or may give the same results as one travelling at 1250 fps. Also, shooting same type bullet from a pistol chambered for .38 and one chambered for .357 Mag may or may not have the same results. Why, because the .38 pistol will have less free travel from case to barrel than from a .357 pistol.