The obvious answer is the other car charges can't really put out 1 amp, regardless of what they say on the label. There is no way to actually tell what power the phone is consuming, short of building an ammeter shunt cable. But I suspect when you pull high amps (near 1 amp) the voltage falls off.
When the voltage falls off, the charging circuit has to boost it, and the only way to do that is to draw more amps. So its a vicious cycle.
Plug your Nexus One wall charger in and run all those services and see if it keeps up.
My experience is that it does, no matter how hard I'm using the phone. The N1 wall wart produces a full 1 amp. And it is certified to do so by a third party. Its a Linear Power Supply, which means it keeps the output voltage constant. (Usually done with a
78xx voltage regulator chip). These things are rock solid stable.
Now when you use a high amperage charger, you never tax it to the point that it can't maintain its voltage (you are running it at half power so to speak), so your N1 can pull a true 1 amp all day long. The two amp charger has no problem keeping up.
The phone's internal regulators won't let it pull more than an amp thru the USB ports, so its not going to heat up, and you aren't doing any damage.
If the truth were known, you would probably be incurring MORE risk using those cheap-*** power supplies that can't really keep up, because the on board charge chip in the phone has to work harder.