They can do it because not all their patients are there at the same time, and they can sell the patient 2 oz and that patient takes that home and doesn't come back for another month or so, but the dispensary can re-up on that patient's two ounces right away and if somebody else comes by and wants to buy it before the patient comes back, that's fine and legal, it's just easier for the dispensary if you make them your caregiver, but if the patient already has a caregiver they're happy with it's harder on the patient.

And it seems like some dispensaries both grow their own and buy it from other dispensaries. If they do that they don't have to worry about scheduling their cropping just right to barely meet the demand (and let's face it, demand is more than what 3 flowering and 3 vegging plants can keep anyone in continuous supply of), they can crop out with this patient's two ounces, sell it to the patient and then turn around and immediately buy another two ounces, because now the dispensary's inventory is only so much when their patient count allows the inventory to be larger.

Probably the biggest reason that there's actually an ample supply of medicine is that patients can fill out a change of caregiver slip as many times as they want to, they can do it every day theoretically if they want to. Filling out change of caregiver slip on its own doesn't take away your previous caregiver's right to count you as a patient. Legally the police must prove that your old caregiver was notified in person, in writing or over the phone that the patient would no longer require the caregiver's services in order to bring charges against your old caregiver. So there are lots of people who do just go around filling out a change of caregiver slip at each place they go, which effectively allows each caregiver to cultivate enough product to meet that patient's needs, otherwise if patients and caregivers were kept strictly to the plant limit in this state there would not be sufficient medicine for anyone. The plant count in this state would need to be increased considerably if it's ever going to be taken seriously.