Excel 2007 : IRR function

I have some financial statements and one of the things I was requested to do was using the IRR function. I do not understand exactly what it is suppose to measure but it has to do with cash flows. I tried highlighting all the Cumulative Net Case Flow to use this function but the value I got back was 25.94% when I was suppose to get back 29.42%. I should get this from my profitability sheet how do I use this formula to get this value. I have attached my worksheet.