Why do you think it should be 11%?

What are you doing with the returned principle - reinvesting, holding, or withdrawing?

You have 1000 invested for 66 days ($16.46 return) and 1000 invested for 21 days (no return). If you weight that out for an annual return for both it comes to around 7%? XIRR is probably not the best thing to use for such a small sample...