两个选择都会产生相同的结果，但有一个我无法为DataFrame赋值

Question

请考虑这个例子：

import numpy as np
import pandas as pd
df = pd.DataFrame([1,2,3,4,5], columns=["col"])

df[df["col"] == 3]["col"] = 11 # Does not work.
df["col"][df["col"] == 3] = 55 # Does work!

虽然分配的结果不同，但基础选择会产生相同的结果：

df[df["col"] == 3]["col"] # Looks like the same as
df["col"][df["col"] == 3] # this

为什么一种方式有效，另一种方式无效？