The problem is that the prediction is based on the past, not the future.
This is a pity. There should be an app that predicts the distance based on your PAST record, on the ebike model, and on the PLANNED tour. It should interface with the bike's sensors, which is not impossible. Something like ABRP for electric vehicles, but with the additional uncertainties related to your current physical conditions. But the results would be much better than the current ones.
Standard disclaimer: If you are going to write that "it's impossible", "it would be useless" or "it can't be done", think twice, because this is the mantra with EVERY substantial innovation, from google search to spotify to the iphone etc....