There is a human looking at the photos. When you take the Eurostar you can see them sitting in a booth after the gates. If you look into the booth as you pass, you'll see a grid of photos on screen. I'm assuming they're overseeing it all rather than checking everything manually.

I noticed that these gates were quite slow ID'ing me the first couple of times I went through them but now I hardly have the time to see my face flash on the screen before the gates open. Maybe there's a human check on initial passes. Either that or their code has gotten way better or they store data that helps matching after a few uses.