Regarding being able to explain (This is a reply as much to OP as this, as they both bring up the concept): Are you sure you're not testing for memory and verbal skills vice technical ones?

I've built a lot of cool shit! For the projects I've worked recently, I can talk your ear off. For one I haven't touched in a few years? I will sound like I don't know what I'm talking about, unless I reviewed it recently, or the right memories were jogged!