Each lender will have a note. It's important to check that the LMI calculator can be used for your scenario.
CBA premiums have a note next to them explaining that you must use the CBA LMI tool for self employed, refinance, cash-out, rental properties, or FHO
CBA also has a Geographical Loading which can apply for certain postcodes. This should be confirmed before moving to the Apply Online stage.
The reason for this is because some LMI Premium Calculations are dependent on the loan type, and some are just too complicated and varied to be represented in Mercury, so the lender calculators need to be made use of.