perhaps we are both wrong, take the rotary quiz from turbo.discovery.com
http://turbo.discovery.com/quizzes/rotary-engines.html
personally, i have always considered the displacement of an engine based on the assumption of a single combustion per combustion chamber in the series instead of per rotation (i think that's how i mean it).
effectively, power only comes from one firing per chamber per rotor, not two or three, even though two or three are part of the system (depending on how you count). if we're going to count by combustion cycle, than the displacement would increase.