I found out, that codespell gives wrong answers in case of a tab is used within a C/C++ string. For example:
test.c:

int main()
{
    std::cout<< "\tTest\n";
}

dict.txt:

ttest->test

Scanning with test.c with codespell gives a wrong result:

$ ../codespell.py dict.txt test.c 
test.c:3: tTest  ==> test