I don't know much. But maybe it is (partly) due to its pro-rata calculation.
It is explained here:
http://en.wikipedia.org/w/index.php?title=Interactive_Brokers&oldid=114578306#Commission_and_fees
You don't get discounts for all contracts. Each tier of contracts are charged differently.
For example, a customer executes 1,500 US futures contracts in a month, the IB execution costs would be:
* the first 300 contracts at $0.90
* the second 700 contracts at $0.70
* the third 500 contracts at $0.45
Fees are charged on a monthly basis, not pro-rata basis. For example, a customer opens its account at 20th. The whole sum of any monthly charges will still be applied.
It is explained here:
http://en.wikipedia.org/w/index.php?title=Interactive_Brokers&oldid=114578306#Commission_and_fees
You don't get discounts for all contracts. Each tier of contracts are charged differently.
For example, a customer executes 1,500 US futures contracts in a month, the IB execution costs would be:
* the first 300 contracts at $0.90
* the second 700 contracts at $0.70
* the third 500 contracts at $0.45
Fees are charged on a monthly basis, not pro-rata basis. For example, a customer opens its account at 20th. The whole sum of any monthly charges will still be applied.