Even reading the dungeon master's guide it doesn't explicitly say they should be doing 6-8 combat encounters a day. It states that a party can handle 6-8 medium to hard combat encounters, then states that if the combat encounters are easier the amount they can handle goes up, and if the combat encounters are harder, the amount they can handle goes down. You can very well satisfy the adventuring day experience calculations with just 2-3 combat encounters, or throw in some non-combat encounters that can drain resources like traps, puzzles, and environments (though there isn't much guidance in the 2014 books on how to factor that in, which they should include in the 2024 books in my opinion).
All in all, the book is referencing how much can be thrown at a party before needing a long rest, and used a mix of medium/hard encounters as an example but not a hard and fast rule for how a game should be run. Revisiting the example the 2014 DMG uses, four third level characters, they can handle 4,800 XP worth of monsters in a single day. Assuming you're running 3-6 monsters per encounter and using the encounter multiplier table, that could just be three encounters of two hard combat encounters and a medium combat encounter, with 250 XP left over.