The issue isn't with the specific phone or whatnot. The problem is with bluetooth technology in that it wasn't originally built for streaming the amount of data such as those found in high quality audio files. What happens is the data is compressed, sent through bluetooth to your car, then it's played. When you connect via USB, that type of data loss isn't present (I would assume the data transfer/stream isn't 100% lossless, but it's not to the degree of bluetooth).
In other words, bluetooth audio is playing compressed files while USB or 3.5 mm audio jack connections are generally playing the uncompressed files. Quality will generally always be worse on the bluetooth streaming. You can read more about it here if you're interested:
I personally think the convenience of never having to take my phone out, having the music start playing automatically when I start the car, out-weighs the slight audio quality lost but I can see why people want top quality audio from their car's sound system