Lingo,
I suspect the "delta" is a reference to its symbolism in maths and engineering, ie "change". The AAC would've been familiar with the expression ever since they did groundschool on the Gazelle - Aerospatiale and subsequently Eurocopter have always loved including delta measurements in engine/instrumentation parameters.
Mind you, it does come over in this context as a bit of "exclusive", management-styleeee jargon.