The silo will get more slots in a future update, it just didn't make it into this one for some reason. The reason it doesn't make sense is because it is supposed to have another level that hasn't been released yet. It is going to have more slots in the future. The fort probably should have been at PH8, but its not. Oh bother. Looks like you have to upgrade your PH to be able to have everything in the game. Who would have thought you'd miss out on content by not maxing your base out?