Structural and magnetic properties of nanocrystalline compounds of the general formula La_1-xMnO_3+δ have been investigated combining x-ray diffraction and x-ray absorption spectroscopy in order to achieve an accurate description of the sample atomic structure in relation with its magnetic properties. The study reveals the segregation of different phases instead of a homogeneous sample composition. Interesting is the segregation of a vacancy-doped phase La_yMnO₃ whose composition is the same (y~0.9) for all the investigated samples, independently of the average sample composition. Since this phase is found to be principally responsible for the magnetic properties of these compounds, such a phase separation explains the largely composition-independent magnetic properties observed in the range 0.9>~La/Mn>~0.7.