G/f is finishing up her degree with art history and this question came up.
I found this article: http://artdaily.com/news/7839/Van-Gogh-Museum-Acquires-Gustave-Caillebotte-Work#.WSClJ2iGM2w
But it didn't really go into why the museum is choosing to host these pieces from other impressionists, only that they are underrepresented.
I am actually curious myself.
I get the impression that you are overthinking this. They simply organise exhibitions from works by artists who worked in styles similar to those of van Gogh. Currently there is an exhibition on 19th century printmaking so there are a lot of works on display by Toulouse Lautrec, before that they had an exhibition on Daubigny and a few years and a few years ago they had an exhibition on Munch. So it really fits with what they normally do.