I don't know if harmonics are an issue here, but if they are, doesn't the skin effect increase with increased frequency? If, for example, all of the A phases were next to each other, flat, the one in the middle might carry less current than the other two. Also, if improperly grouped, the proximity of the metal enclosure might cause a choking effect on the conductor nearest it. These problems could also occur at fundamental frequency.
I dunno!
