As you know, the disability and the child-rearing provisions allow you a drop-out period when you're disabled or when you're taking care of young children. It helps you protect your eligibility for some benefits, and it also helps you increase your average. As you said, it's an average of your pensionable earnings.
The enhancement is not an average per se. The enhancement is based on what you accumulate in your best 40 years. The social insurance portion of the CPP remains in the base. The enhancement is linked to people's contributions. It's like a top-up to your retirement or other survivors' pension or disability pension, so you get your base CPP, which will cover your flat rates for your disability, your flat rates for your survivors, which are paid by contributions from all. Your top-up is linked to your own contributions.