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.

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.