It's quite common for manufacturers to put something extra in Canadian seats that isn't present in US seats in order for it to pass a standard we have called compression-deflection testing. Basically, any area that a child's head could contact in the seat, has to give a certain amount, and bounce back a certain amount, when a device is pressed against it.
The very base level of the Radian in Canada has extra comfort foam that the radian 65 doesn't have. I know there are other seats in Canada - such as the My Ride, Nautilus, and Frontier XT that also have additional material in the head area. I believe the original MA had something different about the foam as well.
Sometimes it may be something as simple as density of foam being used, or the foam being slightly thicker.
There are also some actual dynamic testing differences between countries as well. The differences will be less once manufacturers have to comply to the new standards starting January 1, 2011, but they will still remain. For example, CMVSS 213 will require that all harnessed seats be tested with a lap belt, UAS, and a lap/shoulder belt. The US FMVSS 213 standard does not require the lap/shoulder test. And that is actually a pretty significant thing when you consider most vehicles in use have a lap/shoulder belts in the position a car seat is installed in...