The Dungeon Master's Guide presents guidelines for gauging encounter difficulty based on the Adjusted XP of monsters faced. The Adjusted XP is the sum of the monsters' experience values, modified by a factor based on the total number of monsters in the encounter. Eg. an encounter against 3-6 opponents uses a multiplier of 2, meaning each monster's XP value is doubled when considering the difficulty of the encounter.
However, the XP awarded to the players is still the unmodified sum of monster XP values. It seems odd to me that while a larger encounter is more difficult in terms of Adjusted XP than two smaller encounters of an equal total size, the XP actually awarded remains the same.
Why is the Adjusted XP not awarded to players at the end of an encounter? Answers must be based on citations from the designers of DnD 5e.