NEW YORK ( TheStreet) -- It's a mystery that matters: Some lenders will approve your mortgage application, others won't. How come? And how to you find the ones most likely to say yes? If it were simply a matter of one person lending to another, it would be easy to understand different results. One lender might have more money on hand than another, or one lender might know you better or simply have a more trusting nature. But since the financial crisis, about 90% of new mortgages have been backed by a government entity: Fannie Mae, Freddie Mac or the Federal Housing Administration. Those organizations set the underwriting standards, which are rules that determine whether an applicant gets approved -- things such as the borrower's income related to the size of the loan. Lenders must abide by those rules or they can't sell mortgage-backed securities to investors. So if the rules are standard, why the different approval decisions? minimum credit score for approvals. Some lenders therefore use 640 out of 850, others go as low as 580. Lenders may impose tougher standards, says Guttentag, to avoid the problems that can follow after a borrower stops making payments. Too many such incidents will undermine the lender's ability to get the backing it needs from Fannie, Freddie or FHA. "In addition, loan underwriting includes some judgment calls, exposing lenders to the risk that their judgment might be overruled by the relevant agency," Guttentag says on his website, The Mortgage Professor. "The adequacy of property appraisals is an important example. If Fannie Mae or Freddie Mac determine that an appraisal is unsatisfactory, they will make the lender buy back the loan, while FHA will refuse to insure it." So lenders with different judgment calls make different approval decisions. BankingMyWay's and LendingTree's present your initial request to a number of lenders. The responses will narrow the field, with your best choice likely being those with the cheapest combination of interest rate and fees. To improve your chances at a good loan, consider postponing the process until you can boost your credit score. You might, for instance, pay off car loans or other debt, correct mistakes on your credit report or wait until any late payments are further in the past. A key factor in winning approval is the size of your loan related to your income. You have a better chance of being approved for a smaller loan than for one that's right at the limit. So you could buy a cheaper house, or scrounge money for a larger down payment. Finally, consider the size of the loan related to the appraised value of the property. The higher this loan-to-value ratio, the riskier the deal for the lender, because it would be tougher to sell the home for enough to cover your debt if you were to default. So you have better odds of approval with a 20% or 30% down payment than with 10% or 15%.