Now, it's natural to take that as a standard or even minimum due to the fact that single encounter days are usually pretty obvious and so the players will burn excess resources to make it even easier than the standard ease of EL=level "challenges," and a DM who runs site based dungeons "intelligently" will also push doing as much as possible before resting (because the DM will then boost the dungeon). But as long as there was some threat and they expended appropriate, rather than extravagant, resources, a single encounter day is perfectly reasonable and not at all against the DMG's guidelines. And defeating a single encounter with restraint, ending the day with nearly full resources, is another potential demonstration of power and mastery, as well as a rest: you fought and were ready for more, but it turned out you didn't even need to worry.
This is separate from table 3-2's Encounter Difficulty breakdown. Though there is also the potential for planning EIHP (easy if handled properly) encounters and having the players fail to handle any of them properly and thus rarely seeing a truly easy encounter, the 10% of deliberately underleveled encounters does not necessarily mean they are intended to be combined in the same day.
And? So they get no xp due to excessively favorable circumstances, but still get treasure and mission completion progress, as well as the feeling of power associated with curb stomping an enemy.and actually takes it even further than I do, for example it flat out says that a fight without resource loss on the party of the players isn't a challenge and shouldn't be worth any XP.
I would further note that single encounters where the PCs expend an extravagent amount of resources should give reduced or even zero xp, as they create for themselves an excessively favorable situation (and in-world it makes sense too, because when you need to learn and maintain the skill of measured resource use, overkill is just dumbing yourself down).