If anyone is familiar with this function, can you help me with it?

I have my values in column B, and my dates in column A. If I enter the
"guess" as 0.1, then my XIRR value is returned as 0.099. If I enter my guess
as -0.1, then then the XIRR value returned is entirely different.

I had thought that the guess only sped up the calculation, and never
affected the outcome. Am I wrong?

Thanks
Daniel