I think you may either be mistaking how the displacement math is interpreted or formulated. I'm not saying that from a scientific basis but just from observations of my own..
I have friends that blast their extracts in a large shop with all the doors open and they smoke weed steady in there with no explosions or fires. I AM NOT SAYING THATS GOOD TO DO!! So the oxygen must displace the butane with not too much of a an issue with that much fresh air. They don't use fans at all either. I thought they were fucked for doing so but they've been doing it like that now for a month or so.