If you're being processed in Mississauga, they give you two options for your passport: a) if you are abroad, send to the original VO which would have processed your application, had you not been kept in Mississauga; or b) if you are in Canada, you can send it to Ottawa. For Americans, Ottawa is the VO they would have been processed at, so with all the Americans and everyone in Canada sending their passports to Ottawa, it does end up being a good chunk of the CPC-M COPRs that get processed at Ottawa. However, like you said, there is also a chunk who end up sending it to the other foreign VOs, depending on where you are from