Maybe the Charter has a tighter bore than the Smith so pressures were slight higher and resulting velocity more, or maybe the Smith has underrsized cylinder throats (they seem to be famous for it). When your cylinder throats are undersized as vs the bore size, the bullet is swaged down under pressure, then when it gets to the barrel, it's undersized and doesnt seal as well so velocities are lower. This condition also usually affects accuracy, so if I'm correct, your CA would be more accurate than the S&W. (Probably).
Slugging the barrel and having the cylinder throats reamed to .001 over bore size corrects this and usually results in better accuracy with all bullets also.
I'm no gunsmith but this is my understanding after having read up on the subject some. I'm trying to get a smith to shoot where it so far will not. I measured my cylinder throats and they are ~.355 (or less!). Maybe someone else will chime in who has more than a basic understanding of the issue.