When using XIRR to calculate the internal rate of return for an investment of $10.000 that after 2 year has become $12.100. I'm expecting this to be 10%

If I use XIRR to calculate this for the 2 years period 2010-2012 it return 10% as expected but if I change to 2011-2013 it returns 9,98566%:

A1:=DATE(2010;1;1) B1: -10.000
A2:=DATE(2012;1;1) B2: 12.100

XIRR(B1:B2;A1:A2)=10%

BUT

A1:=DATE(2011;1;1) B1: -10.000
A2:=DATE(2013;1;1) B2: 12.100

XIRR(B1:B2;A1:A2)=9,98566%

Why?

Peter