On the surface, there wasn’t much separating D.C. United and the Chicago Fire heading into their match at RFK Stadium last night. Both teams had scored and allowed the exact same number of goals — 20 for, 17 against. Both held large margins in the way they score, namely, from the run of play — United had scored 70% of goals in open play, the Fire, 65%. United had not lost at home, while Chicago…