Why can I hear both tracks when gain set to zero & crossfader in middle?

I have recently downloaded Djay 2 to my Iphone 5S and I have noticed that when mixing two tracks with the crossfader set to the middle and the gain level set to zero I can still hear the track I want to mix in. Can anyone help this novice out or is just a bug. I have version 2.7.2. Thanks.

The gain level cannot be used to “kill” the sound, this can only be done through the volume line fader. However, the line faders are currently not available on the IPhone’s UI (only on the iPad version) due to limited screen space. On the iPhone version, the volume line faders can only be controlled via a supported MIDI controller with line faders.

I’m still pondering this one…can someone please tell me what I’m doing wrong.
I mix out a track taking the gain down to zero, but can still hear this track coming out of the phone speaker (it is very quiet though. I have the cross fader set to the middle. Thanks in advance for any help.