need help explaining the question

4) Jack and Jane both use the statin drug Crestor, which is marketed by AstraZeneca (AZ). Both obtain the drug from the same Walgreens pharmacy. Jack is enrolled in a small HMO; Jane has no insurance and pays for AZ on her own. Pricing of drugs like Crestor is complicated by a system of warehousing and rebates, but at the end of the day, AZ makes less money from Jack's prescription than from Jane's prescription. Why is it that a small HMO can secure for Jack a better deal on Crestor than Walgreens is able to secure for Jane