This was my experience when I first got it, exactly what you say: The weight of the battery on top of the mod is heavy enough to fire the button. Now it doesn't happen, and I have absolutely no idea if it's because the fire button has become less sensitive (maybe dust or whatever creating more friction / resistance?), or if it's because my muscle memory has me holding it slightly differently. I think it's that the button has actually lost a bit of its sensitivity, because I do typically hold it as you describe, and with the button in crook of my thumb, held sideways with the weight of the mod bearing down from above (standard palming style of mod hold) it doesn't fire just from that any more. ¯\_(ツ)_/¯
The thing is, if the change is environmental, one cannot depend on it: dust, humidity, dryness, temperature might make a (slight but enough) difference in the sensitivity, but that's not dependable or calculable.
The thing is, if the change is environmental, one cannot depend on it: dust, humidity, dryness, temperature might make a (slight but enough) difference in the sensitivity, but that's not dependable or calculable.