We define a plane configuration as a system of $p$ points and $l$ straight lines arranged in a plane in such a way that every point of the system is incident with a fixed number $\lambda$ of straight lines of the system and every straight line of the system is incident with a fixed number $\pi$ of points of the system. We characterize such a configuration by the symbol $(p_{\gamma}, l_{\pi}) $
-page-95/sect 15.,chap-III Projective Configurations, Geometry and the Imagination by David Hilbert
Why exclude those configs where each line is incident with different number of points and vice versa?
Note: I think there is a typo of writing a $\lambda$ as $\gamma$ in the text.