Umm, how about -ence words show an abstract quality, and the -ency words show an instance of that quality in action.

Thus, The judge's lenience was criticised by the popular press. We're talking about the judge's general attitude.

The defendant was grateful for the judge's leniency in sentencing him. We're talking about a particular instance.

Similarly, dependence is an abstraction, while a dependency is a territory which is dependent on another. For example the Crown Dependencies (the Channel Islands and the Isle of Man).

Bingley

Bingley