The President's Choice mussels in sauce are (properly) fully cooked and then frozen in packages for your convenience. I can't find heating instructions for their products but, regardless of which method you use, they should be prepared from frozen with the goal being to heat them - not cook them.
Regarding the live PEI mussels, you are overcooking them. If the meat is shriveled, they are overcooked. You can't just assume that 5 minutes is the ticket for properly cooked mussels.
While there are different methods of cooking mussels and other shellfish such as grilling or baking, for steaming, there are some tips to help ensure that the mussels are fully cooked but not overdone.
The reason for letting them sit for a minute or two in the residual heat is that some mussels will open very quickly, ~3 minutes. That typically is not considered enough time for them to be fully cooked. And if they take longer, an extra minute or two in the residual heat is not enough to ruin them.
Last but not least, remember that time can vary. Even with mussels that take longer, it's still a quick process. So it's important to keep your eye on things.