This is likely because only 1 dimm per channel is supported at 1066. If you run 2 dimms in single channel, it likely downclocks it to 800. If you use 2 in dual channel, you have 1 dimm per channel.
Yes, that is it exactly Falcon.
And they run "unganged", which is not, as some think, is the same as single or dual channel.
If you run them "ganged", they will also default back to 800mhz no matter which 2 slots you have them in.