There is a lot to it, but in short. Here is an answer to a question similar
1: It is a sigmoid curve, all vaults earn. Less fast to start, increases up to average, levels of at 20% above average. So huge vaults massively over average are not linearly earning more, vaults are incetivised to get just over average, but not more.
2: Rate is 2 step,
a: The rate per X Gets, X is calculated by network to increase safecoin when there is a space deficiency and decreases this X on over abundance of space is available.
b: Rate above is regulated by the Sigmoid curve
3: More than network average is to ensure there is always space to store, bit not too much.