There is a disconnect between the accepted dimensions of one of the commonly used Canada Post services and the equivalent USPS size.
The result is what you see.
The workaround seems to be to fiddle with the dimensions (increasing them by a centimetre or so) until the new listing shows the rate correctly.
This doesn't cause any Real Life problems, because the 'fake' dimensions are still on the same rate card as the real ones.
I don't know which service it is, because mostly I use Flat Rate (and free*) shipping for my products.
*Free Shipping means that the cost of shipping is included in the asking price. It doesn't work for every seller, but buyers love it.
