As documented, the [p, S, mu] output of polyfit centers and scales the returned polynomial, so it returns a different polynomial than the [p, S] ouput. If you want the same polynomial as Excel, use the [p, S] output.
As for the 
value that excel gives you, matlab does not give you that straight, but as explained by John here, you can calculate it:
value that excel gives you, matlab does not give you that straight, but as explained by John here, you can calculate it:[p, S] = polyfit(X, y, 2) %if you want the same polynomial as excel.
r2 = 1 - (S.normr/norm(y - mean(y)))^2
