Actually looking at the VOAs here, I wouldn't be calling this anything like a switch to magnesium. This is still a primarily calcium package, with a decrease, for whatever reason, in calcium, and still a fairly insignificant amount of magnesium, at least in comparison to what you'd find in a real magnesium oil, such as certain E7, E9 HDEOs.
In any event, retention is the real issue. Starting TBN is just that - starting.