Mortgage Overpayment - Lump sum or monthly? I've been playing around with mortgage overpayment calculators on moneysavingexpert. 

If I overpay my current mortgage (2.44%, 33 years remaining) by 10% today, £16k, I will supposedly reduce my total term by 4 years and 8 months. If I instead choose to spread that £16k monthly and overpay my mortgage by £40 monthly (£16k divided by 396 (total months in 33 years)) I only shorten by 3 years and 1 month.

I want to ask, why does overpaying in a lump sum reduce the total term by a lot less than overpaying monthly? Overall, the same amount is being overpaid, the only difference is lump sum or monthly. 

Thanks in advance.