I *think* the miles to go is calculated from the shortest path from aircraft -> active waypoint, and then as per the rest of the active route.
Say you are flying on left downwind, and go abeam the CF for your landing rwy - the CF is 5 miles to rwy, and the CF is 3 miles off to your left hand side.
FMC should say distance to go is 8, but that won't include the turn you will have to make on to finals (which in this case will always be more than 3 miles - you can't make that tight a turn!), as well as any further distance you may fly downwind.
Hope that makes some sort of sense
NW3 (757/767)