Every lender stress tests differently
No lender assesses your loan at the rate you will actually pay. They add a buffer on top and then ask whether you could still afford the repayment. That buffer is set by each lender, within regulatory guidance, and the difference between one buffer and another compounds across a thirty year term.
A one percent difference in the assessment rate can move borrowing capacity by tens of thousands of dollars. It is not a reflection of how risky you are. It is a policy setting.
Living expenses are benchmarked, not just declared
Lenders compare the expenses you declare against a statistical benchmark for a household of your size, income and location, and they use the higher of the two. Two lenders using different benchmarks will arrive at different surplus income for the same household.
This is why a careful, accurate expense conversation before an application matters more than most people expect. Understating expenses does not help; it simply produces an approval you cannot comfortably service.
Existing commitments are treated inconsistently
A credit card with a $20,000 limit that you never use is still a $20,000 liability to most lenders, assessed at a monthly repayment of several hundred dollars. Some lenders assess HECS-HELP debt due to finish in two years as a permanent commitment; others take the remaining term into account.
The same applies to investment property income, overtime, bonuses, rental yield shading and how a partner's income is included. Each of these is a policy decision, and each one moves the number.
What this means for you
The highest number is not automatically the right one. Maximum capacity and comfortable capacity are different figures, and the gap between them is where financial stress lives.
The useful question is not 'how much can I borrow' but 'how much should I borrow, and with which lender, given what I want to do in three years'. That is a structure conversation, not a rate conversation.