Two tokens divide one promise
USDu is the unit meant to move around one dollar. It does not rebase and its holder does not receive strategy income merely by waiting.
A user who wants the income deposits USDu and receives sUSDu. The share price rises only when a rewarder transfers earned USDu into the staking pool.
That accounting boundary matters: market price can move independently from the staking exchange rate, so USDu peg performance and sUSDu strategy performance answer different questions.