Actually, the act doesn't allow for it currently. The Chief Electoral Officer used his power under section 17 of the Canada Elections Act to adapt the act for the last general election.
Bill C-76 would make it an official rule that the counting of the vote for advance polling can begin one hour before the close of polling on polling day. Traditionally, when this power has been used, it has been when a large number of ballots were cast at the advance polls. I think that one of the rationales for this was that when the results of the votes are made public on election night, often the results for the advance polling stations come out very late because the vote was longer and the number of votes was much higher.
That being said, on page 104 of the bill, in lines 17 to 19 in English, the returning officer can only count the vote at an advance polling station if he “has obtained the Chief Electoral Officer's prior approval” for doing so.
This is an authorization by the Chief Electoral Officer, and the counting is done in accordance with the Chief Electoral Officer's instructions, so this gives flexibility to the Chief Electoral Officer to determine which advance polling stations should see the counting of the vote begin in advance of closing.