I have a situation to explain, one that boggles my mind.
I DM for a group of friends, right? We've been having at this campaign for months now, and the PCs just crossed the plane into high-level territory, where their power dramatically increases. The group consists of a Level 13 Paladin, a 15 Wizard, a 13 Sorcerer and an 11 Ninja (all humans). They are all equipped with stuff that is about in line with what is expected of characters of their level.
As a storytelling-oriented DM, I will from time to time pit them against a powerful being as a great way to cap off a long adventure or story arc. At my DM's planning table, I designed a straightforward encounter, one that the PCs were not expected to win. I pitted them against a 20th-level Rogue and a 19th-level Fighter, and I had expected them to make successful Sense Motive checks to determine that these opponents were stronger than they were (the Paladin has max cc ranks in Sense Motive and almost always comes through for the PCs on this front). I expected them to run, as these two characters were stronger than usual. I fully equipped these characters with gear equivalent to what a 20th-level NPC should have according to the DMG table. I even hand-picked their magic items so that they would stand a better chance against the two casters and their patented "Twin Disintegrate of Death" move, in case they didn't flee.
Not only did my PCs pummel the daylights out of these higher-level NPCs, they did so without taking more than maybe 20 damage each. I was stunned, and awarded everyone but the Ninja full XP (because the Ninja's win against a 20 Rogue is considered a "stroke of luck" according to the XP table; I did give him full XP for slaying the 19 Fighter). Needless to say, they all gained a level and are well on their way to their next one.
Tell me: is this sort of thing supposed to happen? I can only chalk it up to numbers and a poor Fort save by the rogue against a Disintegrate. It seems almost crazy that the NPCs fell so quickly without even putting up much of a fight, but the worst part is that now my whole planned and plotted story arc is thrown into disarray. What I'm asking is, Did I do something wrong or is this just another example of the "CR = class level" system gone awry?