The scale in CC3 is arbitrary. If you set a scale of 1 unit = 1 mile and then change it to 1 unit = 1 km, then your map will be in kilometres rather than miles. You'll have reduced all of the measures on the map as you'll have redefined the base units. CC3 measurements are measured in "units". It's the drawing properties that make these a real world value. When you measure stuff with the ruler, it multiplies the number of units by the value given in the properties (IIRC).