I have get a strange result when use the Pandas.Series.mean()

Question

I want to calculate a mean value of a pandas.series object.

But I get a strange result...

code like follow:

import pandas as pd
ds = pd.Series([1,1,2,3])
mean = ds.mean()
mean2 = (ds - mean)/mean
mean2.mean()

result:

2.7755575615628914e-17

so, why the computer do this?

thx.