Red,
The answer is that the climb rate and angle at the early oei segements is set at a higher power, so the poorer climb efficiency of Vtoss is sometimes offset by the greater power.
VTOSS is used in the initial climb segment, so is predicated on 2 minute power (or 2.5 minute power, depending on the engine rating scheme). The ROC at Vy is predicated on a lower power, usually 30 min max contingency power (or 30 min oei power, or max continuous power, again depending on the rating scheme that was selected by the engine manufacturer).