Sure. The first and obvious point is that there is a delay between the time when inflation is registered and the time that benefits go up.
The bank did a study looking at people's perceptions of inflation versus the actual number that is published. The things people buy differ. Seniors are going to buy different things than families with children, who are going to buy different things than people going to school.
The CPI is for all Canadians. It's not just for a particular group, so it's certainly possible that for seniors relying potentially more heavily on things like groceries, the prices are going up faster than the general level.
The CPI is an average of a variety of prices. It doesn't necessarily represent everyone's experience. In fact, it represents no one's experience because no one both rents and buys houses and buys cars every year. It is an average, so people can experience it very differently depending on where they are.