It comes from model derived values, which in turn come from ride data, using a 2 month rolling aggregate of ride bests.
So if the data doesn't contain enough all out efforts the model derived values may be too low.
I'm not really sure there is much we can do about that though.
I assume that the user will know how representative the data is and therefore how accurate the model derived parameters are likely to be.
I could add an Endurance Index derived from configured values though, but as a metric and not an estimate ?
Mark