In response to the Federal Housing Finance Agency’s (FHFA) 2025 acceptance of VantageScore 4.0 for mortgages sold to Fannie Mae and Freddie Mac, Milliman conducted an in-depth analysis of the implications of “lender choice”—the practice of allowing lenders to select either Classic FICO or VantageScore for underwriting and pricing on a loan-by-loan basis. Using loan-level data from more than 45 million mortgages originated between 2013 and 2023, our study demonstrates that lender choice introduces a systematic bias to default rates, resulting in higher default rates within a credit score cohort. Default rates are estimated to be approximately 30% higher on average under lender choice compared to Classic FICO. This bias on default rates, without corresponding adjustments to pricing, exposes mortgage credit providers to greater risk while simultaneously reducing guarantee fees. This paper provides critical insights for mortgage professionals navigating the evolving landscape of mortgage underwriting and risk management.