Does os.path.join work strangely?

Question

Python

Hey!
Somehow strange (first join) os.path.join works:

import os

path1 = '/var'
path2 = '/tmp'
path3 = 'tmp'

print(os.path.join(path1, path2))  # /tmp
print(os.path.join(path1, path3))  # /var/tmp

Why is that?