Why is repr(int) faster than str(int)?

I am wondering why repr(int) is faster than str(int). With the following code snippet:

ROUNDS = 10000

def concat_strings_str():
    return ''.join(map(str, range(ROUNDS)))

def concat_strings_repr():
    return ''.join(map(repr, range(ROUNDS)))

%timeit concat_strings_str()
%timeit concat_strings_repr()

I get these timings (python 3.5.2, but very similar results with 2.7.12):

 1.9 ms ± 17.9 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)
 1.38 ms ± 9.07 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)

If I'm on the right path, the same function long_to_decimal_string is getting called below the hood.

Did I get something wrong or what else is going on that I am missing?

update: This probably has nothing to with int's __repr__ or __str__ methods but with the differences between repr() and str(), as int.__str__ and int.__repr__ are in fact comparably fast:

def concat_strings_str():
    return ''.join([one.__str__() for one in range(ROUNDS)])

def concat_strings_repr():
    return ''.join([one.__repr__() for one in range(ROUNDS)])

%timeit concat_strings_str()
%timeit concat_strings_repr()

results in:

2.02 ms ± 24.3 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)
2.05 ms ± 7.07 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)