Okay.. Let me try to make it as easy as possible :)
1 Cuban Convertible Peso, always has a value $1.08 $US. It's fixed by the Gov't of Cuba.
The value of Canadian money in Pesos, varies based on the exchange between the $US and the $CDN. Beacuse the Canadian Dollar is almost at par with the US dollar right now, the exchange rate is Almost equal.
You pay a bit more, when you buy Pesos, and get a little less when you sell; because the exchange office has to make a profit as well.
Okay, now the catch.
The Cuban government, is trying to expand their collection of US bills, so if you take $US bills into Cuba to exchange for Pesos, the Cuban gov't takes 10% of your money right off the top, and you only get to exchange what is left, for Pesos.
So, if you exchange $100 US it means that you are only getting credit for exchaning $90, because the Gov't takes the other $10, before they convert the rest.
This does Not apply to Canadian bills, so you will receive credit for the full $100.
So, even if the exchange "rate" is better for the $US, you are still losing out, because of the 10% right off the top.
I hope this clears up the confusion. :)