As an interviewer, I try to ask a consistent question in a consistent way, so that I can calibrate my feedback against all of the people I've interviewed before. It's nice that you claim to have some previous code, but most of my candidates aren't able to share their code, and some hopefully small number of my candidates are going to submit someone else's code and claim it as their own. I have worked with and interviewed people that can discuss all the things, and run through other people's code as if it's their own, but are unable to write their own code.
That said, exceptional candidates are exceptions and maybe should be handled outside the normal process.
THIS. I have 8 years of open source work in bugzilla and github. If companies really give a shit, they would review code you already wrote and ask questions about your body of work. Not some arbitrary test.
Also, why is no one willing to look at code I have already written?