confusing chair

I love this chair but it confuses me. I don't understand why it doesn't have a center back slat. The back posts are intact so it's not as if there had once been a slat there that broke off. And the top slat is curved at the top, so the chair wasn't designed to have a woven back. It's actually comfortable to sit in, but it looks a little odd to me. If anyone could explain this oddity of the 'missing center slat' I'd be interested to know.
Most of the chair is oak but the back slats may be of a different hardwood -- I can't tell for sure. I think the 'mushrooms' on the front of the arm rests may be walnut or maple. It's a typical adult-size chair, though a little on the small side. It is very light for its size. It's very sturdy. I don't know anything about its history; I bought it at a 'junk' shop in upstate New York, about an hour west of Albany.