I had the same problem with my company, I use to work 400 hours/year of OT. After I asked they ended up changing the pension plan document to define earnings as base pay to specifically exclude OT, our plan document just said earnings so I asked them if that was the CRA definition of earnings (includes OT) or something else. It is up to the company, not up to you or anyone else, there is almost no chance of getting retroactive pay because its sort of gray.
I'm now on the companies pension committee, if you do ask make sure its done very politely and you too might get tapped for the pension committee (sometimes they can't find anyone to fill the roles, its like being part of the chess club). Your company will also have a pension committee, I think its mandatory. Basically its a group of people who are tasked with making sure Sun Life isn't scamming them and they probably meet 1-2 times/year.
Better questions to ask yourself is are you invested appropriately. DC plans often have better MER's than private investment, you should balance your investments with that in mind.