“Average“ means there are a few who make millions and the rest make like 40-80k. JK Rowling would still be considered a writer, for instance, pushing the average up for her few movies.
Another issue is that it tends to be very bursty. They will often get a big payout, but then nothing for several years. There aren't that many writers who get a reliable pay check, so it is more (in common with acting) a lot of people getting occasional big payouts and then drifting off to be replaced by the next batch of hopefuls.
I doubt average amounts are in any way meaningful in this context.
Also, if I am understanding things correctly, that's gross pay which means Uncle Sam will be taking a fair old chunk plus they're effectively self-employed which means agents, managers, lawyers, etc. will also be taking a chunk. One page I saw suggested ~50% of it will go in tax and expenses before the writer even sees any.
What we really need to know is median pay per year but I suspect that's going to be a difficult set of numbers to find.
While what you say is possible, I would think there could be some members who are not working very often making much less than 40K, having the opposite effect.
I doubt average amounts are in any way meaningful in this context.