If you look at their statement you can roughly decipher that they're saying the average working writer makes over $200,000 a year. But that "working writer" part is cleverly obscured in their phrasing, giving the impression that an average writer takes in a healthy six figures annually.
And that, of course, is what's been published, repeated, and swallowed whole. Look here and you'll see it reported as fact by ABC: Average. Writer. $200K.
And that's why it's so important that you read this. It's a brief but brilliant distillation of what a total snowjob this is, why even the word "average" is inaccurate. What we need to be talking about, if we want the real picture here, is the median income because, as the piece cites:
"if Bill Gates walked into a homeless shelter, the 'average' income would skyrocket, but it wouldn't change the fact that everyone else is poor."
So what is the median income for a WGA member? Go read the article -- but first you might want to get something soft to cushion your jaw when it drops.