That's because there's close relationships between "free (as in freedom) software" and "open-source." Neither is a proper sub-set of the other though.
Bolded part has me curious. What sort "free software" would not fall under the looser defintion of "open source" at the same time?
My recollection is that there's some edge case non-GPL stuff that FSF is ok with, but OSI had issues with, but I don't recall exact examples, and my google-fu is failing. I did find this from the FSF: The term “open source” software is used by some people to mean more or less the same category as free software. It is not exactly the same class of software: they accept some licenses that we consider too restrictive, and there are free software licenses they have not accepted. However, the differences in extension of the category are small: nearly all free software is open source, and nearly all open source software is free.
So for the most part the free software stuff should fall under open source, but the FSF clearly feel that there's some subtle cases out there.