Also, since the last known position (just under 2 hours into the flight, 18:22 UTC) is fixed by the radar, the fact that they flew faster before that point does not put them any further out.
Ah thanks. So the assumption that has changed is the amount of fuel left at the point of that last radar fix. From then to the last partial ping is a fixed time interval so you have to adjust the speed to get the right fuel efficiency for the remaining fuel to last that length of time. Then you can compute speed multiplied by time to get the distance.
I'm sure it's more complicated than that but is that roughly the idea ?