Thanks, you're absolutely right. If I visit /subsite1/subsite2/admin/site-map to create the subsites, then they do inherit from that subsite.
That's even worse than I thought.
Okay, then, if I don't hear any protests, I'm going to change this, so that it inherits from the site node immediately above it. That's the only thing that I think I'd be able to explain to users, and it's the most logical and reasonable solution I can think of.