My guess (and remember, only the admissions committee actually knows) would be major development cases or kids who have such extraordinary and exceptional circumstances that the adcom believes that the student has promise and potential far beyond the low grades and scores and so the they decide that the student will be one of a handful they take a risk on. I’d be surprised if merely being a good athlete were enough. There are plenty of top notch athletes with far better numbers.
And all that assumes the graph is accurate. If the data didn’t come directly from Harvard, you can’t know.