I've noticed a tendancy in the Scaling sidebar to skip over a very important part of the game, and that's this.
What if your group isn't scaled for the number of people on the adventure?
Most of these adventures are designed for 4 characters of level X.
However, most groups tend to have 5-8 PC's in them. It is my recommendation that you focus on this more, as a Dungeon adventure that's difficult for four is usually, at the same level, a cakewalk for six. But if you adjust the character levels even slightly, a PC dies, because their saves just aren't as good. So you get a higher casualty rate because XP and Treasure distribution is wider.
(Except, for some reason, Prison of the Firebringer, which ATE my group and turned them into meat chunks. Out of 7 PCs, I got a total of five deaths. It was pretty heinous. If not for revivify, more people would be dead.)