why would I reduce performance to match? makes no sense, if one card can perform better than the other than it should.

I'm running both cards at a performance level that they can comfortably maintain, one is performing better than the other due to its age of wear and tear and that is true be it thermal paste, chip or other physical components that have not worn out as yet.

"If one doesn't overclock as high as the other, that could be caused by any number of things." yes that's true, and one of those number of things is wear and tear caused by many years of active use. Maintenance aside, the unused card will almost always be superior.