The real math that describes the behavior of fusion reactors is too complicated to actually carry out, so physicists have to make some simplifying assumptions. They try to choose assumptions so that the results they get are still valid, kind of like when you say, "Keep the change" at the grocery store. You assume that giving up small amounts of money on occasion won't make you go bankrupt, even though you're not actually adding it up all the time to keep track of how much you've given up.
It turns out that one of the simplifying assumptions that physicists have been making over the years does change the result. Giving up the "spare change" does make you "go bankrupt." In this case, ignoring small-scale turbulence makes you lose a significant amount of heat. But figuring that out required some really hairy math and a butt-load of computing power.
