I found a great recipe for Chilles Rellenos - the first time I made them - they came out perfect. I used pasilla Peppers and the spice/heat was just right. Since then, I have not been able to duplicate the heat level no matter what type of pepper I use. I'm removing the membranes and seeds - and the skin as I can - it's really hard to do. Other than that, the only difference I can think of is that when I made them the first time it was fall and now it's winter. I tried pasilla, poblano, anaheim - in all cases - they're not just a little hotter - they're excruciating. (to the point where just working with the peppers has my HAND burning a day later - and of course the Rellenos were too hot for anyone to eat, even after roasting and frying them.) All these peppers are listed as being "mild"... What gives?
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 & 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.
Chile peppers, even of the same variety, vary in capsicum content from farm to farm and even from plant to plant. Some peppers are even vary in hotness on a single bush. Pimentos de Padron, for example, are relished as Spanish bar snacks specifically because 1 in 8 is an unexpected burst of hot pepper. That's why Scoville ratings of various peppers are always such a broad range.
So in addition to Zanlok's excellent answer, you should just in general expect poblanos to be more or less hot each time you buy them.