Pandas copying wrong values

[vmarg]

Can someone explain me this?
Python 3.7
pandas 0.25.3

import pandas as pd

foo = pd.concat([pd.DataFrame({'foo': [10,20]}), pd.DataFrame({'foo': [30,40]})])
bar = pd.DataFrame({'bar': [1, 2, 3, 4]})

foo["bar"] = bar["bar"]

print((foo["bar"].values == bar["bar"].values).all())
print(foo)

Output:
False
   foo  bar
0   10    1
1   20    2
0   30    1
1   40    2