It's just plain old risk assessment by Boeing.
They don't take reverse into account for normal take-offs but it is still available should you reject.
If the eng with the serviceable reverser fails you no longer have that option so they use rated thrust. ie accelerate distance to V1 is shorter therefore stopping distance is longer.
Of course you only have this luxury when you would have done a red thr take-off. A field length limited full power take-off wouldn't have any additional margin beyond those mandated by the perf requirements.