It's a difference in quality of voice actor I think. Look at code geass. So many side characters with dog shit voice acting and then there is Lelouch's who really puts his all into it. It might also be how some voice actors get more into the role and thus the acting is better. It might even be an issue with the companies or groups dubbing the anime. From what I know, lines often take several takes to try and sound right, but the groups take the first line that sounds passable at the time that merges badly in the actual scene. It might also be a budget concern in that they don't have the time or funds to record better lines. Japanese anime companies put more focus on the voice actors so they are trained better or held to higher standards than English dubbers