my guess (and that's all it is) is that when you've had your failure (I think it's calculated at MTOW) your single-engine FL will be lower than the one you'll have an hour later and lower still than the one two hours after.
so even on one engine you'll have (normally) climbed after one hour and higher after two.
so your airspeed will have changed and so the disrtance will not be just simply three times 415 kts.
hope it's clear (and that it's right)
seb