I asked GMAC about some of these edge cases with very low score and here is the official response 😬
As a computerized adaptive test (CAT), GMAT scores are computed using an algorithm that takes into account not only the correctness of responses and unanswered questions, but also the response patterns and the psychometric and statistical characteristics of each question, such as question difficulty and discrimination power. Incorrect answers contribute as significantly to the GMAT scores as correct answers, leading to score variation despite equal correct responses. As a result, it is normal to observe a wide distribution of GMAT scores (and percentiles), even with the same number of correctly answered questions. Furthermore, it is no secret that the test includes some questions not for scoring but for field testing.
At the section level, GMAT Focus test takers with one incorrect response in the Quantitative Reasoning section typically score around 87. However, in very rare cases, this section score can range from 77 to 90 depending on factors like the difficulty and discrimination power of each question answered incorrectly or correctly; the score attained is an accurate reflection of the test taker's performance. Keep in mind that individual section scores are just one piece of the GMAT Total score. Percentiles associated with each score are based on mean scores across all GMAT test takers - while scores remains constant, percentiles may change with each annual calculation of the mean.