> It's interesting to note that '9' is ascii 57, and ':' is 58.

Right;  it's that part that convinces me that the problem is somewhere deep in _Py_dg_dtoa, in the guts of the float to string conversion, where a xxx999999... result is rounded up to xx(x+1)000000... incorrectly.  And it seems at least possible that there's a compiler bug somewhere, since it's happened before.  It's also possible that there's a bug in _Py_dg_dtoa;  this seems fairly unlikely (but certainly not out of the question), since this bit is pretty much exactly David Gay's original code.