Bishop of Binding says:
When Bishop of Binding enters the battlefield, exile target creature an opponent controls until Bishop of Binding leaves the battlefield.
Whenever Bishop of Binding attacks, target Vampire gets +X/+X until end of turn, where X is the power of the exiled card.
The card has a ruling that explains:
If there is no exiled card as Bishop of Binding's last ability resolves, most likely because Bishop of Binding left the battlefield while that ability was on the stack, X is 0.
The situation I'm confused about is why that would be the case if a valid card did go to exile and then left exile. For example, let's say you exiled a creature with it and then used Processor Assault to put it from exile into their graveyard.
My confusion comes from the rule stating when you should use last known information:
608.2h If an effect requires information from the game (such as the number of creatures on the battlefield), the answer is determined only once, when the effect is applied. If the effect requires information from a specific object, including the source of the ability itself, the effect uses the current information of that object if it’s in the public zone it was expected to be in; if it’s no longer in that zone, or if the effect has moved it from a public zone to a hidden zone, the effect uses the object’s last known information. See rule 113.7a. If an ability states that an object does something, it’s the object as it exists—or as it most recently existed—that does it, not the ability.
I've highlighted the bits that are relevant. As far as I can tell, from this rule, it should mean that the ability continues to work with the same value of X even once the exiled card has gone, as long as the Bishop sticks around. Why isn't this the case? Am I missing some other rule?
Another example would be Isochron Scepter.