Originally Posted by
positron
Watch for galvanic corrosion (happens between alu. and steel) around the rim/nipple interface. not a huge deal, but some drops of oil can help prevent it... more of a problem where there is rain/salt on roads.
That sounds like the most plausible explanation. All the nipples are the same dimensions, they all get wet at the same time, the corrosion progresses at the same rate for each nipple.