The probability density function (PDF) of a global measure in a large class of highly correlated systems has been suggested to be of the same functional form. Here, we identify the analytical form of the PDF of one such measure, the magnetic order parameter in the low temperature phase (critical) of the 2D XY model using a quadratic (spin-wave) approximation. We present strong evidence that this pdf describes the fluctuations of global quantities in other correlated systems. These include the famous Wolf's sunspot numbers for both ascending and descending phases, nonlinear prediction of the Paiva riverflow and the re-scaled positive and negative fluctuations of daily returns of stock prices and indexes.