Thanks for that helpful info, csglinux! I just scrolled down to check out my iTunes purchase history and was surprised to find that the "iTunes" card that I had was used up when I purchased something from Apple (I purchased Scrivener when I had an $8 balance, having no idea that that balance was going to be used towards something I purchased from APPLE (I thought my gift cards would strictly be used for iTunes, but its actually for any Apple purchases). When I redeemed my new $10 Gift card just minutes later, I saw my balance go from $8 to $10, not understanding what had happened to the $8. Now I understand. I was also surprised to see that in my "Purchase History", my monthly Pandora charge is also going towards my purchase histories (I had no idea that Pandora was somehow linked to my "Apple Purchases", but there it was in the history! Good to know!). So yes, many of the balances don't update immediately, but have been long used on other iTunes OR Apple purchases that you may have made (including Pandora in my case). And, just as you said Linux....your balance can easily end up going to exactly zero by making a purchase which depletes all the available credit and then draws the remainder from the credit card which is linked to your account.
I hopes this helps other out there who are having the same question! 🙂