getNumInputChannels and getNumOutputChannels in Reaper with AUs

In Reaper, with an AU version of my plugin, whenever I call AudioProcessor::getNumInputChannels() and AudioProcessor::getNumOutputChannels(), it *always* gives me 1 input and 2 outputs, regardless of what the actual inputs and outputs of the track really are. I made sure to call those methods during or after prepareToPlay().

This problem does not happen with VSTs, because in that case getNumInputChannels() and getNumOutputChannels() just return the maximum number of inputs and outputs specified by my configuration macros (ie,  8 inputs and 16 outputs). This is fine because I can just ignore the extra inputs and outputs, but in the case of the AU, I cannot invent more inputs and outputs...!

I also have the same problem with buffer.getNumChannels() in processBlock; in the AU I always get 2 outputs, while with the vst I get my maximum value. 

Any idea what is going on here?