I too have noticed this with my Canon iPF6100 and Canon Fine Art Water Resistant Canvas. To fix it, I just changed the size of the image in photoshop. So if I'm doing a Canvas that will end up being 60x20, hence my print is 64x24 with 2 inches all around for stretching, I just change the image size to be 64.25 and make sure my "constrain proportions" is unchecked. I also would create a paper size to be 64.25x24 and I pretty much get a print this side.
But here is the thing. When it comes to stretching, the print now ends up being too long. If you are just wrapping around, no problem, but but for a mirrored edge or just a black edge, you want to make sure that your "seams" so to speak line up. And after you stretch your canvas, (i make mine pretty tight), the 64.25 ends up being too long. So now I just print 64x24 as is, and even though it is a little shorter on the long end, once I stretch with my canvas pliers, it stretches back out quite nicely and ends up being within 1 or 2mm exactly where the edge of the stretcher bar is. Hope this helps.