While there may be a 100% anilation with matter-antimatter reactions the resulting thrust is not perfectly efficient due to energy consumed as the rest mass of charged and uncharged pions, energy consumed as the kinetic energy of the uncharged pions (which can't be deflected for thrust); and energy consumed as neutrinos and gamma rays.
I'm unsure if the resulting efficienty is 60% however, not all of the energy produced in the anilation is converted to "stuff" that is useful for thrust.
I'm unsure if the resulting efficienty is 60% however, not all of the energy produced in the anilation is converted to "stuff" that is useful for thrust.