Hi,
I take it that you have not yet applied for PR?
Unfortunately, if WES evaluates it as a Postgrad diploma than that's what it is according to Canadian standards. Sorry for that. How much less points will you get for that degree as it now is considered a postgrad diploma.
To make you abit happy, I had a friend who went to Spain and studied a bachelor degree in business administration. The degree was 5 year long. But when she returned to Sweden they evaluated it to be nothing more than a college diploma, which is only 2 year long here in Sweden.