This is done with separate bonus links. The bonus link that you see on the right will only provide the army once all capitals are captured. There are also other 'hidden' bonuses that provide armies for each bonus separately like the bonus 'London'. These can be found by navigating through the bonuses menu on the left when you're previewing the map. A nice way of bypassing the problem.
But I may be wrong, I'm not a map specialist after all.
Edit: These bonuses don't require bonus-links? Great!