MP Dizzy?

Quick question on total advance.

When say, 15 initial, 20 mech and 15 vacuum, come together (if they ever do) it adds up to more than half of the distance between towers, with 25 deg in the distributor. What keeps the previous tower from pulling the spark?

Does it have anything to do with the fact that the distributor rotates half as quickly as the crank? Crank degrees are different that distributor degrees.