What if you reframed the question? Rather than thinking about this from the buyers standpoint consider the builder. If both the preferred lender rate and an outside lenders rate were the same it should not matter to you whether you use their lender or the outsidelender. But now imagine that the preferred lender has a buyer of the mortgage that is willing to pay a premium for that loan. They might then offer a small discount knowing that they are going to be very well paid on the backend because they have a good buyer for that mortgage.