Headphones vs car aux

A simple google search did not come up with results, so I apologize in advance if this is common knowledge and I did not dig enough to find the answer.

When I plug my phone into my aux cord in my car the volume is always at max - the last setting I had it at in my car. When I plug it into my headphones though it is at the same level I previously had the volume at when using my headphones - a level that wont make my ears hurt. This is exactly how I want it so that the volume isn't blasting in my headphones when I plug them in and start playing, but how exactly does my phone keep track of this?

 

Ah that makes sense and is exactly it. Thanks! I had always wondered how this was done.