all the retails' orders might be re-route to other bigger broker or fund, that is why their brokerage fee could be so low.

basically what these type of broker houses do is firstly, they attract customer to open trading account with them, then direct all the customers' order to US side designated market maker to match huge order flows. Market maker will make tons of money by making bid-ask spread from the order flow provided by such brokerage firms, and the brokerage firm in return will get big amount of compensation from the market maker for providing the order flow, or the business.

therefore, this type of brokerage firm makes money mainly on the "order flow rebate" from market maker. they don't really need to have high brokerage fee as main income source.