I'll be honest, this doesn't seem particularly telling one way or the other. If there was more data about family income/wealth, race, and sex, it might more sense.

As others have said, highly prestigious schools with specific admissions formulae are likely to produce alumni with the money and knowledge to help their kids also get into that school. It isn't totally fair, but unless the admissions specifically gives legacy students a greater chance, it's a more subtle pressure.

Certainly the difference between the schools based on legacy status seems somewhat suspect, however.