That's an interesting thread on MF.
One of the lessons is that to determine a new CL, some analysts will look not just at someone's highest CLs, but also at average CLs. So having $10k-ish limits, then getting a bunch of low-limit store cards, can lead to lower future CLs than would otherwise be possible.
Although it would not explain your Amex and Discover CLs, Vattene. Your history with store cards let you get great prime limits without as many intermediate steps.
So, assuming the dude over there is what he says he is, it sounds very much like the real work is done by the computer, and that the basis for the computer's decisions isn't terribly well communicated. That's not surprising, but I wouldn't trust everything he's saying to be true, even about whatever bank he works for -- not because he's lying, but because they don't tell him everything, and the mechanisms are complicated.
It's well known that the limit is determined by the other accounts you have, but it's not clear whether the bank is calculating a maximum limit, and then deciding how much of that to offer based on your other accounts. For instance, they determine they'd lend you $10K. They look at your collection of store cards with $23 credit limits, and say "he'll be thrilled if we offer him $55!", or they sell your collection of bank cards, with $20K limits, and decide "we have to give him all ten". Store cards are supposed to be reported as such, and banks know that people get them for various reasons (including just to get the intro sale), and that the limits are pretty meaningless. Most people aren't so obsessed with limits as people here and myfico are, so if the limit is big enough for the purchases they're going to make, they're happy. Showing a good payment history is good, too, and there's evidence that number of payments made on time is consideration. That's absolute number, not just a percentage. 100% of 200 payments is better than 100% of 100. I don't know the number at which it becomes meaningless, but for people trying to establish credit it matters.
 We bought some furniture a while ago. There was a discount if you applied for the store's card. So, my wife did. We didn't use the card, in part because the limit they offered was just over what we were spending. (But also because cash back on other cards...) We got the discount.