Or there's UPS, which switches the prominence of the login / sign up links seemingly at random. And does something similar with logging in via a 3rd party auth

Or there's UPS, which switches the prominence of the login / sign up links seemingly at random. And does something similar with logging in via a 3rd party auth vs using your actual 1st party username. I'm sure there's some explanation, like referring page and some cookie setting, but to an unknowing user it's pretty obnoxious.