Why are the constants, I am having are different?
That is the $64 question. Any number of reasons, including
1) data entry error: make sure the data has been entered correctly into the spreadsheet/program/routine that is doing the regression.
2) equation mismatch: you've told us it is a 5th order polynomial. Make sure that is what the regression routine has been "told" as well.
3) general regression error: the regression routine is failing to converge on the correct solution. Debugging this will depend on what regression routine/program you are using to generate the coefficients.