That's just ad revenue, but it is dependent on watch time, video length, and engagement. Somebody with 1 million subs who mostly makes shorts and on their 10+ minute videos there are few comments/likes and people not watching the whole thing will make much less than somebody with 400k subs who mostly makes 30 minute videos that get tons of comments/likes and people on average are watching for very many minutes.
And with that size of an audience, the capacity to attract sponsors who will pay a fixed salary or to sell your own products to viewers is obviously much higher so a 1 million subscriber audience is technically worth a lot more than 20k usd/mo.
Sam's not a genius when it comes to lifting technique or nutrition and I'm certain his cycles involve heavy tren and oral steroid usage, but I have this feeling he's probably fairly smart with his money and not blowing it on nonsense and is set for life many times over. He deserves all the success he's earned as far as I'm concerned.