The introduction of advertising inventory within ChatGPT creates a structural measurement gap that existing digital attribution tools cannot resolve. OpenAI's ad measurement framework operates on a post-click basis, capturing user behaviour after an advertisement is served. It does not measure the organic inference position: the stage within the model's reasoning chain at which brand selection, competitor substitution, or brand elimination occurs prior to paid placement. This paper argues that the organic inference position is the primary site of AI-mediated purchase decision formation, and that brands and agencies optimising against post-click signals alone are measuring the wrong variable. Drawing on CODA (Conversational Outcome and Decision Analysis) diagnostic data across multiple brand categories, we document the gap between AI visibility, organic inference position, and purchase recommendation outcome. We describe the implications for brand strategy, media planning, and the emerging ChatGPT advertising ecosystem.