It does 'jump up and down' according to the boost mode but that is entirely as expected. It is estimating the range you have remaining in the mode you have selected - and you get different ranges in different modes. The golden rule if you want long distances being to run as low a mode as you can manage for as much of the route as you can manage so that you do more work and the battery does less. Turbo being the worst. Eco being the best and Turbo the worst for range.
However it isn't so simple and it will attempt to dynamically adjust the prediccted range according to how well you are doing not just the mode it is in and remaining battery charge. For instance last night I was pedalling in 'eco' mode on the roads until I came to the bit where I turn offroad and start slogging uphill. At the start of the hill (where it had only got my on-road efforts to go by) I switched to sport mode and the predicted mileage dropped from its eco value to 43 miles as soon as I selected sport. Two miles later after slogging up the hill that prediction, still in sport mode, had dropped to 34 miles. However leaving it in sport mode but travelling over easier ground that prediction crept back to 38 miles reflecting the better mileage I was doing.
It does the best it can, but it can only estimate from the information it has, and it doesn't know how tired you will be or how tough the next section of road will actually be
