Help with I Phone 5 & Blue Tooth

Ok, I am new to the Apple World and so far I love it but I can't figure something out.

When I try and listen to the "Music" app via blue tooth in my Prius, I can only adjust the volume with the Iphone.

Why can't I adjust the volume with the volume knob in the car?

I Phone Question

No, it doesnt' make sense but I am going to try using the AUX cable. I am thinking that will work but I would think I should be able to control the volume without touching the Iphone.

More information

Maybe this will help everyone understand my issue.

Scenario 1 - I Phone 5 connected using Bluetooth. I am playing on I Phone with the Music App. Volume on I Phone 5 is all the way up. If I turn volume on car stereo up or down, the volume does not change. The volume only changes my turning the volume up and down on my I phone.

Scenario 2 - I Phone 5 connected using AUX Cable. I can adjust the volume by car stereo!!

Why can't I adjust volume with car stereo volume when phone is connected with bluetooth?