What happens when you disengage the authrottle at a different setting than the one indicated by the levers?
engfirelft well describes what happens when the A/THR disconnects abnormally, as was the case on AF447. The recommended means for a crewmember to disconnect the A/THR is via the "instinctive disconnect" button on the side of the thrust levers. The thrust will promptly match the lever position. Incidentally, I hate that Airbus refers to this as instinctive, on my previous two types this is where the go-around button was located. It's the "counter-intuitive disconnect." But I digress.
Does the thrust respond immediately to the position of the levers?
Yes.
Is the pilot supposed to readjust the thrust levers close to the setting the thrust is currently at?
Yes. When the A/THR is engaged, the thrust levers serve in a limiting function. In the climb detent, where they normally belong, climb power is the upper limit of what can be commanded. Bringing the levers back out of the climb detent will limit the A/THR's authority to whatever intermediate setting the levers are at. With the thrust levers at idle, the engine wil be at idle, even if the A/THR still happens to be engaged. Proper technique is to bring the thrust levers back to match the current power setting, then push the button to disconnect. This is readily accomplished, as the thrust lever position is represented on the N1 or EPR gauge with a donut shaped icon. Match the donut to the needle and disconnect.
Why would a modern aircraft like the A330 not have automatically moving levers?
Only God and Toulouse know. It's cheaper, lighter, and mechanically simpler this way. Really moving levers are a throwback to mechanical actuation of the fuel control unit. The FCU is an all digital function now, there are no cables to drive. Like the rest of the digital flight controls architecture, Airbus decided that mechanical feedback was unnecessary. Unfortunately it removes valuable cues to the crew. One can't hardly hear the engines at intermediate power settings, in order to determine what the engines are doing one must look at the engine display, tactile information is unavailable. It prevents elegant incremental power adjustments while the A/THR is engaged. It adds significant complexity to what should be a straight-forward interface with the aircraft. I love the Airbus, yet myself and many of my associates could imagine a more friendly system. This system does work.