The reason it would be different is that there are two kinds of pension plans that are permitted under the Income Tax Act: defined benefit and defined contribution.
Defined contribution works just like an RRSP. Defined benefits are more complex arrangements that allow you to fund toward a particular targeted retirement benefit.
In a defined benefit pension plan, there's aggregation of risk and there's aggregation of assets. That presents opportunities for more flexible investment policies and also to lower costs. Under the current tax rules, individuals are not allowed to pay for their own defined benefits. An employer must do it.