"Average capital" means that the monthly repayment amount is the same, but the principal paid each month is different, and the interest decreases month by month. The total interest in this way is less than the "matching principal and interest", but the monthly repayment amount in the early stage is higher.
"Matching principal and interest" means that the monthly repayment amount is the same, but the proportion of principal and interest repaid each month is different. The monthly repayment amount is low in the early stage and gradually increases in the later stage. The total interest of this method is more than the interest of "average capital".