Exactly what makes a MOPAR a "Matching Numbers" car?

My 1978 LRT has everything matching except the 360 engine. It has a W in the numbers pad. But it has all 1978 date coded items like water pump, intake, chrome valve covers...

I bought it used from the first owner, He had a invoice from the local Dodge dealer that they replaced the engine with a factory warranty engine due to rear main oil leak.