Progress output gets misaligned with default verbosity

Just noticed that with pytest-xdist 1.22.0 and pytest 3.4.0 sometimes the progress output is off by 1 or 2 characters.

Test file:

import pytest
@pytest.mark.parametrize('i', range(300))
def test(i): 
    pass

Here are a few runs:

$ pytest -n2
...
scheduling tests via LoadScheduling
................................................................................................................ [ 37%]
................................................................................................................. [ 74%]
...........................................................................                                      [100%]
============================================= 300 passed in 3.64 seconds ==============================================

$ pytest -n8
...
scheduling tests via LoadScheduling
................................................................................................................ [ 36%]
.................................................................................................................... [ 73%]
........................................................................                                         [100%]
============================================= 300 passed in 6.23 seconds ==============================================

This problem happens sometimes only, other times (seems random) the output is correct.

Seems related to pytest 3.4.0, because with pytest 3.3.2 I can't reproduce the issue.