probably because the prop is not providing full pass thru of ECN fees and is skimming from the top. You can always go to the ECN to see what their fee schedule is and compare it to what your prop is passing thru to see if/how much you are cheated by your firm.
Example - compare the fees from this firm against the actual ecns or even other firms (prop and retail) to see how they don't pass thru fees w/o skimming:
Also, as a trader, you need to keep up on the fees and constantly make sure they are correct. I have had a few times that the ECN changed a charge and my firm didn't pick it up right away.
