为什么像a*=b这样的就地整数运算比a=a*b慢？

我知道整数是不可变的，因此计算值不会修改原始整数。因此，就地操作应该与简单操作相同，1. 计算值和 2. 将值重新分配回变量。但是为什么就地操作比简单操作慢呢？

import timeit
print("a = a + 1: ", end="")
print(timeit.timeit("for i in range(100): a = a + 1", setup="a = 0"))
print("a += 1: ", end="")
print(timeit.timeit("for i in range(100): a += 1", setup="a = 0"))

print("a = a - 1: ", end="")
print(timeit.timeit("for i in range(100): a = a - 1", setup="a = 0"))
print("a -= 1: ", end="")
print(timeit.timeit("for i in range(100): a -= 1", setup="a = 0"))

print("a = a * 1: ", end="")
print(timeit.timeit("for i in range(100): a = a * 1", setup="a = 1"))
print("a *= 1: ", end="")
print(timeit.timeit("for i in range(100): a *= 1", setup="a = 1"))

print("a = a // 1: ", end="")
print(timeit.timeit("for i in range(100): a = a // 1", setup="a = 1"))
print("a //= 1: ", end="")
print(timeit.timeit("for i in range(100): a //= 1", setup="a = 1"))

输出：

a = a + 1: 2.922127154
a += 1: 2.9701245480000003
a = a - 1: 2.9568866799999993
a -= 1: 3.1065419050000003
a = a * 1: 2.2483990140000003
a *= 1: 2.703524648
a = a // 1: 2.534561783000001
a //= 1: 2.6582312889999997

所有就地操作都比简单操作慢。加法的差异最小，而乘法的差异最大。