While I can't say with certainty, I believe it's intended to shape how players approach encounters.
Say you have 10 orcs in a room, and that this would be a deadly encounter for the given party. They have the option to kick in the door and fight the orcs head on. However, if they're clever, they might employ a ruse to lure some of the orcs out of the room. Now the party will face two easier encounters, but they still get the same xp as if they kicked in the door. IMO, it's intended to encourage "smart" play.
On the other hand, if you award adjusted XP, the party that kicks in the door earns more xp than the "smart" party. There is an argument to be made that this approach is fine, since the first party earns more xp while the second party gets to conserve resources. IME, however, players will follow the xp unless they have an overwhelming reason to do otherwise (such as if the group of 10 orcs is an almost guaranteed TPK).
One possible solution would be to simply award the adjusted XP to either party. You would have to figure out what to do in the event that an encounter shifts outside of its defined parameters though (an orc runs for help and 5 more orcs show up on round 4; do you readjust the xp to account for the new orcs or treat them as a second encounter or ...?). This will also increase the rate at which the party levels, which is something else to consider.