Good summary from rehoult. One point to add is that the actual cost of benefits can (and do) change over time. For example, a benefit might be defined as "the health plan will cover all diagnostic imaging". Well guess what? In most provinces stuff like MRIs are not covered unless deemed medically necessary. You are free however, to go to a private clinic and pay for it.

A pensioner could argue that under the plan, the paid private clinic should be covered.

This is perhaps not the strongest example, but the point is that under the provincial health plans, some things are covered and some things are not. Over time, the list of what's covered tends to change. So if something is delisted, it might be up to the DB plan to now cover it. The problem here is that the contributions to the plan to cover this particular treatment were never made while the employee was actually working. So now it's up to the company to suddenly pay for "new" benefits. Over time, this obviously becomes problematic.