Girls have little inhibition about hitting boys when they are emotionally upset with them, nor do they generally face legal consequences for doing so (except when the violence is lethal). Boys, on the other hand, are taught all their lives that they must never hit girls for any reason, and the current legal system has severe penalties for doing so.
Missing here is whether the survey was restricted to heterosexual couples. Homosexual relationships are notoriously more violent than hetero.