Sometimes the weather affects the heat... Hotter, drier temps = hotter peppers.
Sometimes when the seed provider has grown other peppers near enough to cross... later generations have subtle differences like less heat...
Sometimes... the seed purveyor mislabeled the seed and, we got something other than what was desired.
And... I had a ghost pepper plant that produced amazing peppers for like 5 years, but it's offspring weren't as hot...