클로져를 학습 중에 궁금한게 생겨 질문드립니다

#1

def clos():

  series = []

  def averager(v):

    series.append(v)

    return sum(series) / len(series)

  return averager

avg_clos = clos()

print(avg_clos(10))

print(avg_clos(20))

print(avg_clos(30))

#2

def clos():

  a = 0

  b = 0

  def averager(v):

    a += v

    b += 1

    return a / b

  return averager

avg_clos = clos()

print(avg_clos(10))

print(avg_clos(20))

print(avg_clos(30))

series든 a,b든 변수에 할당하고 수정하는건 같은데 왜 #2번만 nonlocal을 적용해야 실행이 되는 건지 잘모르겠습니다 . ㅠ.ㅠ