There is an issue with the images in the database that affects this marriage registration. This marriage appears in the register on a register page numbered 507. If we examine the previous two pages we see that page 505 lists parties to marriages along with their ages, birthplaces, residences and parents. Page 506 lists witnesses to these marriages and gives dates and locations of the marriages. The marriages listed on page 506 occurred between 21 April and 14 June, 1904, mostly occurring in the town of Cornwall. Unfortunately, in place of page 508, which would have given witnesses and dates for the marriage of interest, there is only another image of page 507. After this duplicate, there are images of pages 513 and 514 in the register. The dates for the first two marriages on page 514 are 27 and 28 Sept 1904.
So, I think that the suggested date in the index came from these other marriages.