Thanks all, for your considered collective responses.
As Biggsy says it's not really a problem - more to do with curiosity on my part - particularly given the marked contrast of filth on the inner side of the chain when on the bike, while the outer remains sparkly.
For the record, the bike has a triple chainset, and very short chainstays.
I'm erring towards Rhys W's suggestion of the rub on the 'next up' sprocket being the cause of wear, and probably also the accumulation of gunk.
By comparison, it looks like the surfaces of the front cage (both inner and outer) are far more smooth and benign. For instance, they don't have the break in surface, as do the sprockets on the rear, thanks to the ramps.
As an aside, how do ramps work? Do the ramps on the next-desired-change-up-sprocket 'grab' the chain to assist it's path onto the larger sprocket? Or are they there to make it easier for the chain to 'leave' the smaller sprocket on it's way to the larger one?
If it's the former, why does the smallest sprocket have ramps? Conversely, if it's the latter - why the need for ramps on the largest sprocket? Or am I totally missing something about how ramps work.
Front mechs usually have smooth rounded stainless steel edges and surfaces that are gentle on the chain......
Not stainless. Of that I'm fairly certain. The cage will snatch a neodymium magnet to it like a rude snatchy thing. Most stainless in this application is austenitic and only very faintly ferromagnetic. Martensitic stainless, whilst magnetic would, I suspect, be way too brittle to be used for a cage.
... Anecdotally, less force (and, depending on shifter, time) is required to derail the chain to the smaller ring than to hold it far enough over to catch on the bigger one.
Is that not more because while you're changing from small to large you are working against the force of the spring? Irrespective of whether we're talking about front or rear changing.
All this leaves me with something of a dilemma when it comes to re-fitting the chain:
Do I reap the benefits of the lower coefficient of friction of Titanium Nitride plating, and fit the blingy side inwards where it appears to rub on sprockets as it changes up?
Or do I leave the blingy side facing outwards so the drive train looks good in photos?