Leakage and non-revenue water

These are not the same thing. Non-revenue water is everything not billed; leakage is only the part that physically escapes from pipes.

Why the distinction matters: A utility can cut non-revenue water substantially by fixing its meters and billing, without a single leak being repaired.
Percentage is a poor measure: Non-revenue water as a percentage falls when consumption rises, even if physical losses are unchanged. The IWA recommends litres per connection per day instead.
Leakage is a residual: It is not measured directly. It is what remains after subtracting everything else from system input, so it inherits the error of every other term.

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System input volume divides into billed authorised consumption, which is revenue water, and non-revenue water. Non-revenue water divides further into unbilled authorised consumption such as firefighting and mains flushing, apparent losses from meter under-registration and unauthorised use, and real losses which are physical leakage from mains, service connections and storage. Only the real losses are leakage.