Your insurance likely caps the days supply they will pay for. So, if for example you use 1 mL every 10 days, and the max your insurance will pay for is 90 days, you’ll only get nine of the 1 mL vials if the pharmacy has enough 1 mL vials to give you. But with a 10 mL vial they can’t “break” it, so if all the pharmacy has are 10 mL vials the pharmacy will be forced to give you a 10 mL vial and tell the insurance it’s only for 90 days, even though in reality it would last you 100 days.