well it appears to be a known problem, as commented in the source of
whitespace-mode.

2014-09-10 2:12 GMT-05:00 Mario Valencia <mariovalspi@gmail.com>:

> sorry, this happens in a text-mode buffer with whitespace-mode on. without
> whitespace mode it only occupies one column.
>
> 2014-09-10 1:55 GMT-05:00 Mario Valencia <mariovalspi@gmail.com>:
>
>> In a buffer, set tab-width to 6. then insert 5 characters, then a literal
>> tab character (C-q TAB). Now you have a 7 column tab.
>>
>
>