You'll find out pretty quickly that they're largely irrelevant.
In combat it's simply an indicator of their approximate range so you can decide if it's worth turning towards them and zapping them out of the sky or not.
Their actual distance doesn't seem to be a major factor as the distances are covered very quickly and the weapons extend your effective range anyway.
Anything off the scanner (wobbling a vague targets) is automatically placed at an indicated range of infinity, to the viewer anyway.
For very close quarters maneuvering, such as threading your way between the spars of a capital ship, the scanner ranges tell you nothing as you're on visual anyway.