Complex sentence: Although electrons are distributed nonuniformly in an atom, an approximation was made that the electrons are distributed uniformly in each small volume ΔV of coordinate space (i.e. locally) whereas the electron density can vary from one small volume to the next.
Split into simpler sentences: Although electrons are distributed nonuniformly in an atom, an approximation was made that the electrons are distributed uniformly in each small volume ΔV of coordinate space (i.e. locally). However, the electron density can still vary from one small volume to the next.