As information...
There are some arm and leg options which are not symmetrical and never have been. For the most part, the indexes of each part appearance represent like versions on one side of the other, but some are not, since the "set" of appearances for that particular index were not designed that way. Most of the Bioware-standard ones are symmetrical, while some that we got from other sources may not be.
The "both" option simply sets them to the same index. There's no guarantee of sameness, however, since each part is a separate resource within the HAK files.
When you get an asymmetric result like that, you may need to change one or the other, because the numbered option may be "null" and not actually correspond to anything. During customization, it can retain the previous appearance until the equipment is swapped out with something else later (when the part will likely disappear) or a valid appearance is selected. If you set to both and the appearances are different, it's a good idea to adjust them both back and forth once to make sure they actually change. If they do, then at least you won't hit the "null" issue.
