This is an oddball. At first, I thought it was a skin problem, but it looks like a core issue.

If you choose Advanced > Structure > Alphabetical or Calendar, added pages, like About.htt or Contact.htt, are ignored. No HTML page is created in the output. It's OK with Flat or the default Mirrored.

Still failing.

I think they should be put in the root, because that's where the skin navigation is typically expecting to find them.

That does introduce some problematic wrinkles, of course. A user might add an "about" page in each of several different folders. But that may not be worth worrying about - a user who is using regular folders in that manner isn't going to choose one of the oddball structures. More important that the basic case works properly.