If you can't replicate it at home with AC bricks... this could be an available current related thing. The apple wall plugs (and apple laptops) are rated to 1A current, and when one of those travel cables is detected the amount of current the phone takes is adjusted downwards to allow for the headset to charge too without blowing through the available current.

It's likely that your car charger is either appearing as a china charger (D+/D- shorted) or is identifying itself as a 500mA charger, and this is modifying the behavior incorrectly (it's likely a bug from the sounds of things).

What appears to be the 1A resistor network is posted here:
http://www.instructables.com/id/Build-a-Battery-Powered-USB-Charger/