A couple of thoughts-
1) Is the unequal division of current consistent as a percentage or as an absolute value? If the imbalance remains 20-25 amp at higher load, I wouldn't be as concerned. If the imbalance increases proportionally and it looks like the conductors will exceed their individual ampacities, then I would be more worried.
2) Are the cables bundled in any manner? Do they pass through the nipple ABCN? Making sure that they are grouped together this way for the whole length might help.
