Let's say someone has 10000 GBP*. Then at Saturday evening. 20000 people* are wanting to share those 10000 GBP*. That's not much left to each person. Then consider some of those 20000 people* have paid to become a premium member of a place, they obviously would want some extras for that.
So now someone has 10000 GBP*. However, they have split it to two pots. One regular pot of 7500 GBP*, and one premium pot of 2500 GBP*.
The same people show up to share those 10000 GBP*. However, 2000* of those are premium members. So those 2000* share the pot of 2500 GBP*, while the 18000* non premiums share a pot of 7500 GBP*.
That's bascially what happens when you download anything from any site in the world. There is a set bandwith, and that is shared among X amount of people. If the number of people is low enough compared to bandwith, is the users own internet speed that will be the limit. If the number of people are too high compared to the bandwith that limits.
* Numbers totally made up, but it is to illustrate how it works in quite broad, simple terms.