personally I still do a lot of analog photography with B&W films, since I have my own Darkroom where to develop them.
To your question: there could be many reasons for that.
The first thing I've thought about is that usually B&W films have problems with the wavelengths around green. They are getting better nowadays, but looking at the date of your photo I don't think those problems where really fixed then. The photographer could have used some colored filter to avoid this, but I don't think he did, since the forest in the background is pretty dark (as you expect it with this "green wavelength" issue). So my first explanation would be that the wood, which was probably covered with some moss, has some parts that are shown darker than they actually are and this makes the people on the photo look "odd".
The other explanation I have for the "oddity" of the people regards the developing time of the film. It seems to me that the film was developed a bit longer than usually needed to increase the contrast (in the foreground it's pretty high). This is not immediately noticeable, but might be strong enough to make the people (with mixed bright and dark clothings) look more defined than the rest. In fact the most "odd" looking ones are the woman standing on one trunk and the man with the head in the very foreground sitting on another one. Notice that they both have dark cloths with here and there some white/bright things.
You might argue that the far background has very few contrast at all and with that you are totally true. But when developing a B&W film if one knows that it was a foggy day or at least the sun wasn't that strong, like it seems to be the case here, one could decide to develop the film longer to enhance the contrast. I've done this several times to get rid of annoying mist, which I didn't want to have in my photos. Despite this, it's now always perfect, especially if there is stronger fog. Anyway you could define this as the "Photoshopping" of the analog photography.
It could be also a combination of the two things I have listed.
I hope I could help you with my answer! Just keep in mind that I made some suggestions based on my personal experience and just looking at the photo you posted. To know the "entire" truth one should look at the negative, since it usually carries much more informations than the printed photo.