Garry,
have you checked that the M straps are all exactly equal in length? Also the check the spreader bars for small missalignments which can subtly effect their length and thus the shape. Control head and weight misalignment is another very remote possibility.
Lastly, if you can find something strong enough, hang the bucket next to the good one and try to see any differences, then fill them and look again. Usually problems are easier to see when they are full, even though the weight wont make let them spin.
Intresting problem. Would love to hear any solutions you can find.