Why is the Howling Abyss Default music still hearable through Dj Sona Soundtrack.

You cant hear it as much during the start of the match due to how the Default music originally is but later on during the match and extremely during a death due to how the beat changes in the music you can hear the Defualt music. Just wondering why the default still plays on ARAM where as you see on Summoners Rift it has been totally turned off and only plays Dj Sona's Music. Wondering if the same could be done to the ARAM map as well.