. . and like other nightshades they cross polinate and come up with new varieties constantly. The only way to keep them pure is to grow them where there's only a few kinds. Soil also causes great variation in hotness.
You can get any hotness you want here, but you have to sample the batch. I've bought Thai chilis that would toast your undies and others that couldn't stand up to a Jalapeno. I've had pobalanos milder than Anaheims and as hot as Jalapenos. Serranos seem to be the most reliable.
One of my favorites is red cherry peppers - two identical looking varieties. Pickle some of the hot ones and sneak them into the salad tray.