I have no idea why you think the result should be 0.2. 0.5630188679245283 looks correct to me with autojunk disabled:

sm = SequenceMatcher(None, a, b, autojunk=False)
total = 0
for m in sm.get_matching_blocks():
	print(m, repr(a[m.a : m.a + m.size]))
	total += m.size

Running that displays every matching block:

Match(a=0, b=0, size=73) '\n#include <iostream>\n#include <string>\nusing namespace std;\nint main() {\n'
Match(a=74, b=73, size=10) '   string '
Match(a=87, b=84, size=1) 'r'
Match(a=138, b=85, size=2) 'i;'
Match(a=141, b=87, size=11) '\n   cin >> '
Match(a=155, b=99, size=1) 'r'
Match(a=160, b=101, size=1) ';'
Match(a=200, b=102, size=10) '\n   for (i'
Match(a=210, b=116, size=9) ' = 0; i <'
Match(a=220, b=125, size=1) ' '
Match(a=221, b=130, size=1) 's'
Match(a=228, b=133, size=1) 'e'
Match(a=230, b=136, size=2) '; '
Match(a=232, b=139, size=2) '++'
Match(a=235, b=141, size=1) ')'
Match(a=237, b=142, size=1) '{'
Match(a=274, b=143, size=11) '\n      if ('
Match(a=285, b=156, size=1) 'i'
Match(a=288, b=161, size=1) 'i'
Match(a=294, b=163, size=8) " == 'i')"
Match(a=305, b=171, size=10) '\n         '
Match(a=315, b=183, size=1) 'i'
Match(a=318, b=188, size=1) 'i'
Match(a=324, b=190, size=14) " = '1';\n      "
Match(a=380, b=204, size=4) 'if ('
Match(a=384, b=210, size=1) 'i'
Match(a=387, b=215, size=1) 'i'
Match(a=393, b=217, size=8) " == 'a')"
Match(a=404, b=225, size=10) '\n         '
Match(a=414, b=237, size=1) 'i'
Match(a=417, b=242, size=1) 'i'
Match(a=423, b=244, size=14) " = '@';\n      "
Match(a=479, b=258, size=4) 'if ('
Match(a=483, b=264, size=1) 'i'
Match(a=486, b=269, size=1) 'i'
Match(a=492, b=271, size=8) " == 'm')"
Match(a=503, b=279, size=10) '\n         '
Match(a=513, b=291, size=1) 'i'
Match(a=516, b=296, size=1) 'i'
Match(a=522, b=298, size=14) " = 'M';\n      "
Match(a=578, b=312, size=4) 'if ('
Match(a=582, b=318, size=1) 'i'
Match(a=585, b=323, size=1) 'i'
Match(a=591, b=325, size=8) " == 'B')"
Match(a=602, b=333, size=10) '\n         '
Match(a=612, b=345, size=1) 'i'
Match(a=615, b=350, size=1) 'i'
Match(a=621, b=352, size=14) " = '8';\n      "
Match(a=677, b=366, size=4) 'if ('
Match(a=681, b=372, size=1) 'i'
Match(a=684, b=377, size=1) 'i'
Match(a=690, b=379, size=8) " == 's')"
Match(a=701, b=387, size=10) '\n         '
Match(a=711, b=399, size=1) 'i'
Match(a=714, b=404, size=1) 'i'
Match(a=720, b=406, size=14) " = '$';\n      "
Match(a=763, b=420, size=1) '}'
Match(a=822, b=421, size=12) '\n   cout << '
Match(a=837, b=436, size=26) ' << endl;\n\n   return 0;\n}\n'
Match(a=863, b=462, size=0) ''

and then

>>> total
373
>>> 2 * total / (len(a) + len(b))
0.5630188679245283
>>> sm.ratio()
0.5630188679245283

give identical results.