This is because by default, the Firestudio headphone outputs are mirrored on DAW output 3 & 4. Most likely you have your DAW outputs set to output to 1 & 2. This is because the Firestudio Mobile actually allows for a separate headphone mix for performers in your studio. If you are looking to create a separate headphone mix, you would need to bus some your tracks in your DAW software to hardware output 3 & 4.

If you would like to hear your mix exactly as you hear it through your monitors, open the Presonus Universal Control software and click the enable button on the phones bus to the right. This will enable anything going through the Presonus mixer, to go through the headphones. A very powerful tool!!!