That's not the point. It doesn't have to be weekly, or even monthly. Whatever the frequency is, Amazon is somewhat likely to already have a package headed to the customer within that interval (much more likely that a dedicated prescription store, certainly).

The advantage is actually greater with less-frequent refills, as the probability of having another package they can piggyback on within an acceptable window becomes higher the less frequent the refills are.

Express Scripts pushes 90 day supply as long as insurance allows it, I assume for this reason.