Looking for financial gurus to share their 1 cent :)i always...

Looking for financial gurus to share their 1 cent

i always thought that placing x$ in 100% offset account will save the same amount of interest as reducing x$ on the mortgage balance.

Please see the below example:

Deposit : 20%
Loan amount : $480,000
Terms : 30 years
Interest rate : 3.20 on both case (outside the scope for this thread's discussion)
Yearly fee : $100 on both case (outside the scope for this thread's discussion)
purpose : first home buyer but may look into other investment purchase in the future (outside the scope for this thread's discussion)

Case A: $100,000 in offset account since day 1 - which will bring down the duration of the mortgage by 5 years (according to ING & any other bank online offset calculator)
Case B: $0 in offset (redraw account), starting balance of $380,000

I know that interest rate and yearly fee for redraw account will be cheaper than offset account - not relevant for this thread's interest query.

Question: in terms of interest payment, would I be better of with case A, case B or both are equal?

Link to mortgage calculator: https://bit.ly/2ViWXoM (or google ING mortgage calculator)

I always thought that case A and case B are equally the same in terms of interest payment. However, after crunching all the number into the above online calculator, it seems that the interest payable is cheaper on case B.

Calculation on Case A
Loan Details
Loan Amount: $480,000.00
Loan Period: 30 year(s)
Interest Rate: 3.20 %
Offset Account Balance: $100,000.00
Offset Account started after: 0 year(s)

Results
Fortnightly Repayments : $957.67
Revised Term : 25 Years
Interest you could save: $125,219.78
Time you could save: 5 Years

Total repayments: $957.67 * 26 fortnights *25 years = $622,485.50 ($142,485.50 interest payable for the total loan)

Calculation on Case B
Loan Details
Loan Amount: $380,000.00
Loan Period: 30 year(s)
Interest Rate: 3.20 %
Offset Account Balance: $0.00
Offset Account started after: 0 year(s)

Results
Fortnightly Repayments : $758.16
Revised Term : 30 Years
Interest you could save: $0.00
Time you could save: Less than 1 Month

Total repayments: $758.16 * 26 fortnights * 30 years = $591,364.80 ($111,364.80 interest payable for the total loan)

I also tested NAB calculator with the exact some result…

https://www.nab.com.au/personal/home-loans/calculators/loan-repayments-calculator#!#nab-loan-calc-results-repayment

Can anyone explain why case A is ended up $30k more expensive at the end of the day?Thanks!