# Different formula of coefficient of determination (R-squared)

I saw the below equation given for "Coefficient of determination" in a paper and thought it must be a typo, then I saw it in another paper too. Would anyone know what this is and how it is different than the coefficient of determination formula I know? Thank you in advance.

The equation of interest is $R^2=1-\frac{\sum(X_m-X_p)^2}{\sum(X_m)^2}$ where $X_m$ and $X_p$ are the measured and predicted values, respectively.

It is the same if you know that $X$ (unconditionally) has mean zero. I gather you rarely know that the true (unconditionally) mean is zero though.