Yeah, this doesn't surprise me. The login system at SO is unnecessarily complex - by which I mean that the complexity of the interface does not match the comple

Yeah, this doesn't surprise me. The login system at SO is unnecessarily complex - by which I mean that the complexity of the interface does not match the complexity of the underlying system, because while both have been redesigned several times they've never been completely redesigned together. So you end up with weird situations like this, where both the interface and the underlying system support multiple associated email addresses, but not in the same ways or with the same functionality exposed. It is... A legacy system, with all that that entails.