Ok maybe not all, but the Nichicon UES series of bipolar "audio-grade" electrolytic through-hole caps have a short lead and a long lead..
The datasheet has nothing to say about it. This fairly recent question: Orientation of "bipolar" capacitors on PCB seems somewhat related, and I did some searching and found a boatload of speculation but nothing that stuck. So far the only thing that seems likely to me is that perhaps the manufacturing process is built around the more common polarized electro with the uneven leads. Or that the reason is common knowledge and I am once again tragically out of the loop.