It's pretty simple, Marxism-Leninism is the largest and most significant subset of Marxism. Much of Marxism-Leninism is just Marxism, and as such it shares with other branches of Marxist thought. When pertinent, it is more useful to use Marxism, rather then Marxism-Leninism, such as discussing the Marxist law of value or theory of the state, neither of which Marxism-Leninism meaningfully deviates from.