These units are customized for each vehicle, attenuating its freq spectrum based on the reflectivity or liveliness of the car they are in, plus, they have noise compensation built in, either reducing the dynamic range so the low level sound rises above the noise floor or just general increase in volume level. They put a bit of fine tuning for each vehicle since they will be selling many of these units.
The drop in units usually are not customized for each vehicle unless you go to a installer with experience on these things.
That is why companies like Blose, a brand that gets laughed off in the home audio world, is so pBopular in the car market. They provide the facilities and ability to mass customize generic stereos for a given car.