I lived in NM from the mid sixties to the early seventies. In those days Hatch chiles came from the Hatch area and they were reliably vey hot. Terroir plays a part, I'm sure, but the seeds have to be a certain variety. If you lived in Albuquerque and grew the seeds from Hatch chiles they would be hot, but you couldn't really call them Hatch. At some point after I left to live in CA a variety of chile called Big Jim was developed and some were grown in Hatch. They were big, varied a bit in heat, but were generally considered mild. If you see a sign that says "Mild Hatch Chiles" you are most likely buying Big Jims. They could be grown around Hatch or they could be from somewhere else.
Since I have not been back to NM in quite a while I don't know what the situation is there now and my information may not be current. The first years I lived in CA I missed real Hatch chiles terribly, and spent the dollars to have them shipped fresh in season. My friend Elaine, also a NM transplant, would also spend hours roasting a giant box of them to freeze for as long as they lasted. After several years of the extravagant purchase the chiles started to taste suspiciously unlike true Hatch chiles I remembered. I'm pretty sure they were mixing in Big Jims in haphazard quantities or they had grown a hybrid. Clearly growers were hip to the value of calling something "Hatch."
