The other 10-20% is spent exhaust gasses. In the real world, the exhaust stroke does not remove all of the exhaust gasses. Yes, scavenging refers to the removal of these exhaust gasses. No, it doesn't refer to only one technique.

No offense, but I think you need to review the Otto cycle basics again. The gas expansion phase happens during the power stroke (3). During the exhaust stroke (4), the piston pushes the spent gasses out into the exhaust manifold.

Turbo scavenging happens during the overlap period, where both the intake and exhaust valves are partially open. Valve overlap is used to help scavenging and to promote cylinder filling. High-RPM motors use more overlap to make up for the decreased duration at speed. But since Diesels are low-RPM motors, there's relatively little overlap. That's where the turbo comes in, blowing more air in during the shorter overlap.

The fluid mechanics of an internal combustion motor are exceedingly complex. A couple of scalar meter readings simply can't describe everything that's going on.