Oh dear - I suspect we are being bamboozled and/or brand jacked.
Not with Schwalbe. Certainly in the Marathon line, I've had some which were different from each other and had justifiable price differences, and the Big Apples too have been on my bikes or in my spares in several different versions, the most useful differences from the cyclist's point of view being the suppler sidewall of the "folding" version (since promulgated across the whole Big Apple range except the racing versions) and various levels of puncture protection. In addition, many forum members have experience of more than one, and some of several, versions of the Marathon touring tyres, differing in the surfaces they're intended for, load bearing, comfort, thorn-proofing (no pun intended), and so on.
Actually, I'm not at all certain that the cushy Big Apple wasn't once ludicrously classed in the Marathon group too, because "Marathon" was at one stage used by Schwalbe to class together long-lasting, puncture proof tyres for commuters and tourers, as distinct from lightweight tyres for roadies and racers, and some of the fatter touring Marathons like the Supreme and the Mondial are themselves balloon tyres, the class the Big Apples rather obviously belong to.
But all of this is the opposite of brand jacking or brand engineering in the old British Motors Leyland sense. Schwalbe includes really different products under an overly large sub-brand umbrella -- it isn't just changing names and pretending it is a new product justifying a price premium, which is what you suspect.
I have to know why "souplesse" has a £25 premium.
Every man must have a hobby. Go to the English Schwalbe technical site at
https://www.schwalbe.com/en/tour and compare, within the type of tyre you're interested in (I've linked the touring tyre pictorial reference rather than the home page, and you need to scroll down to G-ONE), in the first instance the class of puncture protection of which there are several types of different weights and materials and effectiveness, then the formulation of the compound (grouped in classes by wear characteristics), then the sidewall construction, then safety and cosmetic features like reflective bands or colours.
Don't forget, once you're the maximum expert on SCHWALBE G-ONE ALLROUND multi-purpose gravel tyres, to report back so we can be experts too -- or at least less confused.