Hi Maryka - the ascent figure shown during plotting is only approximate, as it is based on fewer readings. The final figure is calculated when the route is saved. This is mainly because the elevation chart is updated very frequently when you plot a route, so it would be very process intensive to calculate the final ascent figure every time and could potentially make plotting unresponsive while it waits for each update.
I've noticed this recently when doing hilly rides -- I will plot a complete route, pull up the elevation line along the bottom and note the total ascent. Then when I save the route without alteration, the finished route page has about 10% more ascent added to it? Why would this be? See screenshots below -- route before saving has just over 800m. Route after saving nearly 900m. Distance identical.
If I edit the route and make a change, then revert the change but don't save it again, it also goes back to the lower elevation number.