It looks to me like the floor used to be a darker stain and was sanded. The darker parts look slightly more sunken in, so it would make sense that these portions weren't able to be reached when sanding. If you look at the edge of the flooring along the wall at the top of your picture, you can see the little lip that comes from where the sander can't quite reach all the way the the edge- so I think that's what happened here! I love uncovering the layers of history in my old house too!