Thank you Sam for explaining this to me. Wow!
The Billboard charts used to be determined by retail sales of LPs and singles as well as terrestrial radio airplay in key markets. It was a very manual process and ripe for fraud (such as the Payola scandals).
Over the decades, retail sales and radio airplay have diminishing roles ini chart position. Now it's all about the streams.
Today, almost nobody purchases music anymore. Most subscribe to some sort of music streaming service where they have access to unlimited content on demand. Typically these services charge around $20 a month, out of which they need to pay royalties to the artists.
This is why the royalty rate hovers around .004 per play. Now even if you download the songs and play them from your local hard drive, the song still belongs to the streaming service (such as Spotify) and they are responsible for paying that .004 each time you play that song offline. (Next time you connect, the play count is uploaded to the provider).
If you ever cancel your streaming subscription, the songs you downloaded from them become instantly unusable. This also occurs if you do not connect to the "mothership" within a certain number of days.
For a $20/mo subscription, you would need to play 5,000 songs in a given month for the streaming service to lose money on your account. (5,000 x .004 = $20)
As you can guess, this rarely happens. Most people will stream less than 20 songs a day - or some 600 songs a month. So the streaming services are able to pay out the artist royalties while still earning some profit for themselves.