I'm guessing you had a forum account and in-game account whose names matched, and that's what's causing your confusion. Despite possible identical names, forum accounts and in-game accounts are 100% different from each other. Kinda like how you can meet two different people with the same name. Just because they're both named John, for example, doesn't mean they're the same person.
Whatever ads you accrue on the forum account can be used to purchase items or services for any in-game account, and you would specify exactly which in-game account you wanted the item or service for at the time of purchase.
For example, I currently have six different in-game accounts and one forum account. If I wanted to buy an MVP card from the item shop, I would use the credits on my forum account, and then tell the item shop which of my six accounts' kafra storage I wanted the card put in.
As long as you know your username and password for your in-game accounts, you have complete control over where things go when spending ad credits.
Did that make sense?