Why have I more stETH than I bought?

Sorry for this noob question, but I am very puzzled… I bought 21.202904822560727813 stETH 69 days ago and now the balance of my wallet shows 21.331163049679926525 stETH.
The only transaction listed is my purchase.
I am therefore wondering where does the difference come from… Any idea?
https://etherscan.io/token/0xae7ab96520de3a18e5e111b5eaab095312d7fe84?a=0xaa07ad6589e9c7fabee984ee11d3c360bb94e947