It's logical to assume the free/open source software disagreement would be over copyleft licenses, then the separation of the two would seem meaningful. But the spirit is mostly a disagreement over the use of the word "free," in 99% of cases both terms mean the same thing.
My understanding of it was that the "free software" movement is making moral claims about how software should be distributed while the "open source" movement is just saying "hey, this stuff is cool and useful" and doesn't have the same kind of uncompromising stance. Which would fit well with the license thing.