Difference between trendline and regression R squared values

Hello

This is regarding the difference in R squared values computed by Excel, while fitting a trendline and while using the Regression analysis using 'Data Analysis'.

We fitted a trendline using the excel graph option for 40 data points and then fitted a straightline with zero constant.The equation came out to be y= 11.837 x, with an R squared value of 0.7384. For the same data set and while using the regression option in Data Analysis, the equation obtained was y = 11.837 x, but with an R squared value of 0.959.

Could anyone please throw some light on this difference??

Thanks in advance

Regards

Hari