|A little summer heat|
The degree of heat of a particular pepper depends on the growing conditions, the soil, and the weather. So the range for the Serrano can be explained by those factors. Also, Serranos will change color on the stalk depending on how long you leave them there. Mine turn red but others might turn brown, orange, or yellow. I use mine mostly to make salsa, along with tomatillos that I also grow. If you need to turn down the heat a little, you can always remove more of the seeds and the membrane from inside the pepper--that's where most of the heat is.
If you've gotten in over your head, heatwise, there are a few ways to get immediate relief. Some proven remedies are: drink milk; drink sugar water; drink alcohol. Some folk remedies are: eat cucumber slices; eat a raw carrot; and, get this, eat more of the same pepper (really??).
Hot peppers are annual in my zone so I'll have to replant if I want more next year. Also, they like full sun and you can easily grow them as a companion plant in a mixed container.
Bookshelf: Container Gardening Books