...
I get the incentive part, and it is normal to work this way, but the way it is described is misleading. When you say 5% interest for staking, I think about 5%, not 10%-20%.
...
If it's misleading, then just think of the 5% as the yearly increase in supply which will be distributed to all online wallets in proportion of the amount the online wallets hold and the time the wallets are online.
Even if you want to hold on to the view of the 5% as the interest received, then one option is to think of 5% as the base return that you will receive from staking and anything above that as a bonus which depends on the overall online network.
I personally like the idea that I am "guaranteed" 5%, but in reality can expect more (an unknown amount) due to the online / offline distribution of XSPEC.
If you start with 10% or 20% as the expectation, you will likely be disappointed in the long term, due to the fluctuations.