I do this when I'm not confident I set up the experiment correctly. If the A's differ bit quite a bit, it's more likely I made a mistake than there's is normal

I do this when I'm not confident I set up the experiment correctly. If the A's differ bit quite a bit, it's more likely I made a mistake than there's is normal statistical variance. I make mistakes daily. I think that's pretty sensible reason for A/A/B testing. Or A/B/B testing. Whatever you like.