How is EDGE explore ETA calculated

Whenever I ride a course I create on the gamin connect web site I find the ETA data field to always be too early. On the website I set the pace to 20Km per hour, but the displayed ETA does not seem to be related to this nor to the current average speed, I've downloaded the Mapdashboard datafield, and the ETA displayed as part of this is always later than the Garmin provided ETA field, and does seem to be related to the average speed.

Is it possible to change how the Garmin ETA field is calculated, or to change the course parameters so that it assumes 20Km per hour?