Pandas: why pandas.Series.std() is different from numpy.std()

Question

Another update: resolved (see comments and my own answer).

Update: this is what I am trying to explain.

Answer 1

I see. Pandas is using Bessel's correction by default -- that is, the standard deviation formula with N-1 instead of N in the denominator. As behzad.nouri has pointed out in the comments,

pd.Series([7,20,22,22]).std(ddof=0)==np.std([7,20,22,22])