It's really about the power the USB port is able to provide and the draw the device is able to pull.
For example (no citations so I could be off on these numbers):
The Iphone 3gs can draw ~650ma from the wall charger Apple provides with the phone which will supply up to 1000ma.
Some/many computers and devices with USB ports limit their power to USB2.0 specs which is 500ma so my iphone would not charge as fast connected to a computer (ignoring the fact? that some devices decrease their pull as the charge nears completion).
I'm not sure of what an ipad can pull while charging but it comes with a 2000ma capable power supply... I would guess it probably pulls ~1500ma while using the supplied power brick. Assuming it does, charging it from a computer or other USB port that supplies only 500ma will work but at 1/3 the charging speed.
So back to the Jeep. I'm not sure what rates the 430N or 730RHB provide, however 1000ma (1amp) is called out in the 730RHR specification here:
It wouldn't surprise me if the 430N and 730RHB supplied 500ma; however my iphone 3gs should charge on the RHR USB port as fast as it's wall charger... but an ipad would only charge at 2/3 rate of its wall charger.
clear as mud?
A unit load is defined as 100 mA in USB 2.0, and was raised to 150 mA in USB 3.0. A maximum of 5 unit loads (500 mA) can be drawn from a port in USB 2.0, which was raised to 6 (900 mA) in USB 3.0.