It's a subtle distinction - but then, hey, this is theology. As Enns points out earlier in his book, there is no such doctrine derived from the Genesis story in Judaism.
[I]t is helpful to keep in mind the crucial theological distinction expressed succinctly by Lutheran theologian George L. Murphy. This distinction is between "original sin" and "sin of origin." The former, as bequeathed to us through Augustine, refers to an event at the beginning of history and requires a historical Adam as the first human to sin and transmit that sin to all subsequent humans. The latter affirms the absolute inevitability of sin that affects every human being from their beginnings, from birth. In other words, Murphy and others counsel that we must remain open on the ultimate origins of why all humans are born in sin (original sin) while resting content in the observation that all humans are born in sin (sin of origin)... the notion of "original sin," where Adam's disobedience is the cause of a universal state of sin, does not find clear - if any - biblical support.
George L. Murphy
As for Murphy, who'd have thought you could be a Lutheran theologian with an Irish surname... isn't there some kind of rule about that? He also apparently has a doctorate in theoretical physics; I just bet he isn't LCMS!
And I'm also led to wonder, not for the first time, whether the world would have been a much better place if that old reprobate Augustine hadn't converted from Manicheanism.