There are way too many what ifs here.
Death recap isn't the most reliable source of information and no one (I don't even think the programmers) can get to the bottom of that code. So just because death report doesn't have the information doesn't mean it didn't occur that way. It only selects the greatest 3 damage sources, but this sometimes isn't true either. And the number of times the damage from the recap doesn't add up to your health pool............
Second, death recap is older than the new masteries, and its quite possible that the champion that got kill credit was running deathfire touch. In which case they might have been burning (aka taking damage) from a champion for four seconds longer than you thought. I don't know if death recap includes the new masteries, but I doubt it.
Thirdly, it could have been any number of buffs that deny'd the execution. Gromp buff (damage over a certain number of seconds) or red buff, liandries, etc.
Lastly, I'm not sure how it decides when those 10 seconds are up. It could be the case (again I'm not sure) that a champion has to not take damage for 10 seconds to be considered out of combat. My example will be grasp of the undying. With this mastery, you can steal health every four seconds so long as you remain in combat. Combat just means you took or received damage. So, the Ekko that was running away might never have escaped damage for more than a 10 second period; resulting in the kill credit going to the last champion that hit him.
