Couple of reasons, IMO. First, I believe they dramatically upped the massive merit scholarships within the past two years (when they cut their class down significantly yet kept the same amount of scholarship money). The stats for avg. indebtedness are for last year's graduating class so it wouldn't account for this. Also, probably less likely, Pritzker may tend to take more private school undergrads who often carry more debt with them to medical school. This also counts towards the eventual "avg. indebtedness".
Personally, I think it's the former. Everyone I've talked to at Pritzker has gotten at least 1/3rd tuition by the time of matriculation and their tuition as-is isn't stratospheric (yes, it's avg./above avg., but nothing like OOS public). That's my best guess though...