They don't, the "rate" is the same. The only thing the vote was for was the wage amount that qualified for the dues. If a 401k was the only source of retirement, it was exempt. And I guess still is. I personally don't think it should be subject to dues.
I get the rate thing but clearly one group is paying more money by virtue of skewed formula. Either way it's biased and I highly doubt it's legal.
Besides, who's to say their A plan is going to be there at retirement? Stupid. I hope it got shot down. If the executive board wants more money then can share mustache trimmers.