The problem is with the Date formats in your data. I mean if you simply copy this data to excel you can see that the dates are intrepeted incorrectly.
I mean if you see 2nd row (6 Months Vs 6 Months) - The Current Month Start Date is interpreted as 9th January 2011 instead of 1st September 2011. The month and dates are swapped and everything is interpreted as January 2011, Hence you since the difference between the dates is marginal. Same is the case with the other dates.
I hope this makes sense!
Cheers - DV