I'm trying to understand it too, as a matter of fact. When the Access to Information Act first was brought in, in 1983, all government institutions that were added to the statute were added full blown--the documents they already had, the documents that had been created before the act came into force, as well as future documents. All the crown corporations had been added--all the documents they held in the past and the documents they create in the future.
The Senate...why they picked the foundations and the officers of Parliament? Those are the only organizations the Senate picked to not have any retroactive application of the Access to Information Act. It came as a surprise to me, and I'm not sure I understand, given the care with which there have been exemptions tailored for all these organizations. If you have carefully tailored exemptions, why then remove all the documents from the coverage of the statute?
So I am not able to help you with a good explanation. I don't understand it.