That's not quite how it works...
It searches for programmes with the same series id, not specifically a next episode id, since there's nothing on a particular programme to say what the id of the next episode will be. When a matching series id is found, the episode id is used to make sure the same thing isn't recorded, so you don't get duplicates.
The episode id should (though I can't swear to it) appear in the now/next data, to trigger accurate recording, so to that extent it is the same data, in the the ids are common, but of course a different EIT table is used to trigger the accurate recording from the one that's used for searching the whole EPG for series links.