Good question. I am in no way an expert on cancels, so take the following with a massive grain of salt. I'm replying to your post because no one else has so far. Perhaps what I contribute may make sense and will be useful.
The first thing that I notice is that the black fades significantly from top to bottom. It doesn't appear as if the person applying the postmark/cancellation used stronger pressure at top and less at the bottom. To me, it looks like it was applied by a watercolor brush. Maybe the ink pad had just had water added. Also, the width of the inside of the line varies.
The following images were created using your image and from a source of several legitimate CSA postmarks:
http://www.jameslee.com/News67/7-10...JULY2013.pdfI approximated the size of the postmark circle on yours to a fairly accurate size:

Here we see that the design fits easily into that circle:

Here's an example of a larger diameter postmark layered with opacity on another type of CSA:

Here's an example of a smaller diameter postmarrk:

I don't know if the design size of these stamps are the same. If they are similar, then I don't see the size of your postmark as being suspect. However, the position is very suspect. We would expect the postmark circle to be towards the bottom left or center instead of the upper side. That is assuming that the stamp was placed on the upper right of the envelope. Did Cancel To Order exist on these stamps?
Despite placement of the postmark, I still feel strongly that the fading of the black ink is suspect. Look at the examples shown in my images and the .pdf resource that I linked. Variations of pressure when applying a postmark doesn't result in a fading effect. It results in an smaller area of ink that is just as dark as the rest of it. I see no fading of darkness on any legitimate example.
I do have to say that the black ink on your example is very circular which I wouldn't expect if hand applied. The inside of that circle is irregular though. Perhaps it was made with a template with a cut out circle instead of a stencil.
As I said before, I am an expert by no means. Take nothing that I have said as definitive.
Perhaps someone else will contribute what they know.
-Edited for clarity