Depending upon exactly where you were starting from and going to your satnav might have calculated the distance as very slightly shorter.
The AA route planner quotes central Enfield to central Glasgow as 401.9 miles and gives the route you've suggested would have been best. However adding a 'via' point of Scotch Corner Services only takes the total distance up to 404.1 miles, using the route your satnav gave you. (i.e. just 2.2 miles further). So, in terms of distance, there's very little difference in it (although the AA site does give the former journey time as 6 hours 37 minutes and the latter as 7 hours 12 minutes).
Chris