Have I mentioned that the encounter levels (EL) for combat encounters is hard to calculate? Just listen to a table after the dust settles and you will hear such things as, “That was too easy!” or “What the heck, that was way too hard for our APL,” or even “Those monsters must be CR’ed wrong.” My personal favorite will always be “Man, didn’t they play test that thing?” It is enough to make even the most level headed person think that all adventure writers are missing a few cards from their decks.
I would like to view this problem from a slightly different angle. I have, in the past, gone over what goes into the encounter level calculations, and the Dungeon Master's Guide has a lengthy write up on how to determine them. What all the calculations in the world can not determine is the party’s effectiveness in combating a certain type of monster, overcoming an environmental challenge or out maneuvering a tactical disadvantage. These things can make a normally hard encounter child’s play or a simply encounter a potential total party kill.
For example, it your party was choked full of archers with flaming arrows, a meeting with trolls might not seem so tough. Try that encounter with a fairly evenly distributed party with no “fire” power and it becomes a serious situation. Almost all encounters can have their ELs lowered by effective use of resources. Burn the trolls, disarm the fighters, silence the mages and use sonic eschewed fire balls on everything else. The more experienced a party and the more resources they have less challenging an encounter will be so don’t blame the authors for your savvy and know-how.
Some parties can also overcome an environmental challenge or even turn it into an advantage. Let’s say you are fighting a band of orcs on a frozen mountain top, the howling wind and snow ripping into your flesh and the snow creating an uncertain footing. A party might decide to think outside the box and create some water to turn the snow under the orc’s feet into ice. Your footing may be treacherous but theirs is worse. Suddenly you have the upper hand and have turned a disadvantage to an advantage. On the other hand, if you still had the party of archers you would be in serious trouble as the wind makes the arrows less accurate. Which party should the author consider when determining the EL? Not as straight forward as it seems is it.
Finally, let say you are at the bottom of a rather tall cliff and a pair of giants is chucking some rather large and annoyingly hard boulders on you and your compatriots. Seems like a serious tactical disadvantage to you and yours. A party with a horn of blasting could collapse the cliff the giants are standing on. Suddenly your disadvantage is a serious advantage; the giants take serious damage form the fall and are now prone at your feet (assuming you have managed to avoid the avalanche of rocks). Is the author supposed to assume that every party has a horn of blasting? That would be sheer madness.
Most adventures are play tested. Many are played more than once but to playtest the adventure at every APL and with a wide variety of parties is not feasible. Most of the typical responses to a challenge are considered but even then it depends on the authors and his (or her) play testers’ experience. Every authors lack the most important element in determining all the variables – you and your party.
Don’t judge an adventure's encounter too rashly because it just might come down to how your party played it. Maybe, just maybe, it was you that made the encounter harder or easier than expected. No matter what happens, reward yourself by taking time learn from your defeats and reveal in your triumphs.