Yes, the behavior is by design.  I think you are misunderstanding how exandtabs() works.  The "tabsize" argument indicates the number of columns each tab position occupies.  So, with a tabsize of 4, the tab positions occur every four columns; the tab positions are where the characters immediately following the tab character start.  Perhaps this example will make the behavior clearer:

>>> '1\t2'.expandtabs(4)
'1   2'
>>> '12\t3'.expandtabs(4)
'12  3'
>>> '123\t4'.expandtabs(4)
'123 4'