Do chile peppers heat vary depending on the season?
Many references indicate that stress on the plant effects the heat rating of the fruits produced. "Good" stress (usually people want to increase heat) is generally a reduction in water supply, carefully and aptly timed, and/or increased outdoor temperatures.

While appearing dated and non-authoritative, this site, also looks very accurate from my experience around growing chiles, and states that (as is common, referring to spice/heat as pungency):


  ...total pungency amount of the medium and mild chiles increase
  dramatically when put under stress.


Normally, chile plants like summer weather (sunlight and warmth), and are not nearly productive in winter. What you're obtaining in winter months may be imported from a very different location, probably from a place on Earth where it was summer when harvested. As indicated in the above-cited article, the truly hot peppers don't get much hotter from stress.

Anecdotal addendum: fwiw &amp; ime, chile plants grown in winter (the ones observed are nagas, tabasco, japone, jalapeños) aren't as productive and the peppers are not anywhere near as hot. So, when not grown in the proper conditions that a species expects - mostly enough daily sunlight or a long enough warm season, the heat in produced fruits can drop dramatically.