I feel like you've defined the point of the mobility study, so I don't believe it ought to be accounted for. The person who earns $100,000 in nation A can now

I feel like you've defined the point of the mobility study, so I don't believe it ought to be accounted for. The person who earns $100,000 in nation A can now comfortably afford a top 10% lifestyle in that country. For example, the very best housing of the country is reserved for them (if they want it: they have first dibs). They now have significantly more economic power than their parents had, at least within their country. That would seem to be less true in nation B - person B might have observed their objective wealth to have increased similarly to person A, but they have been less able to change their relative status in the world and are less able to move up any ladder-based system (I propose that land ownership/housing is typically one such system) and are more dependent on factors outside of their control.