There's something else that you gotta' realize. When it comes to the CME and futures, the exchange tells the FCM that they gotta' have such-and-such in their segregated account or whatever to get into the trade. Discount brokers take it on the chin and offer you very low margins by going on the hook for you.
They are, effectively, using money they have in storage to cover YOUR lack of funds to enter said position.
Don't take discount brokerages for granted in the futures world. They are using their discretion on you, to determine how much of a risk you will be to all the other members in that segregated fund account.
Folks like IB have more money than God, so they can take chances on the riskier types. Like Big AAPL says...I am glad AMP is a bit more strict in their determinations, for it protects the other folks in that pool from the more raucos types.
Of course, I could be typing out of my ass on this, but that is how I reckon' it.