I have the following:

1 Jan 2007 , -10000
1 Jan 2008 , 2000

I get XIRR as -80%. I understand that.

If I have the following:
1 Jan 2007 , -10000
1 Feb 2007, -10000
1 Jan 2008 , 2000

I get -91%. I understand this as well.

But, when I have the following:
1 Jan 2007 , -10000
1 Feb 2007, -10000
1 March 2007, -10000
1 Jan 2008 , 2000

I get 0%. Why?