A few years ago albuterol inhalers were generically available and dirt cheap. However, they came in CFC inhalers. Since CFCs were eliminated a replacement had to be made. Drug companies, smelling exploitation opportunity in the water, realised they could patent these new HFA inhalers and rake in those big brand-name bucks. 

So Ventolin HFA, ProAir HFA, and Proventil HFA were born. They are all the exact same drug from before, but because the propellant is different (with no discernable effect on its efficacy) they are not considered AB rated to the old CFC inhalers or, indeed, to each other.

It's a blatant cash-grab on the part of the drug companies and has led to *sooo* much headache when I have to explain to patients that yes, it used to be much cheaper, but because the US is corrupt and allows pharmaceutical companies (and other major corporations) to exploit the health and well-being of its citizens now it's under patent and expensive again.