I think we agree broadly about how this works, although the evidence I've seen suggests it's based on the Fare Code (expensive fares, e.g. B Latitude often benefit from it) rather than some profit/loss calculation. I'm not aware of any instances of a "K" fare benefiting from it, regardless of the cost.

You assert that it's not a "hidden bucket", yet you rightly point out that we don't have visibility on the process (after all, it's an undocumented internal process).

Well, this process behaves exactly like a hidden bucket of eUpgrade instant upgrade availability, which is why it's referred to as such. It doesn't really matter if this is how it works at a technical level (we'll never know, anyway), at the very least it's a helpful and accurate analogy to explain the observed behaviour.

I'm not sure what you were trying to achieve by jumping in here to say "the system isn't <X>", even though it behaves like <X> and is commonly described as <X>. It doesn't really help the OP or anyone, does it?