You are wrong. The guidlines for creating high difficulty encounters in the 2024 DMG state specifically:
"Spend as much of your xp as possible without going over"
A high difficulty encounter is anything more than a medium difficulty encounter, up until the limit which is the number noted as "high difficulty".
In the 2014 rules it also provides guidlines in two places, first in the commentary about budgeting XP:
"You can build an encounter if you know its desired difficulty. The party’s XP thresholds give you an XP budget that you can spend on monsters to build easy, medium, hard, and deadly encounters."
and second in the guidelines on daily xp budget:
"For each character in the party, use the Adventuring Day XP table to estimate how much XP that character is expected to earn in a day. Add together the values of all party members to get a total for the party’s adventuring day. "
You can claim this should not be used or that the high difficulty number should be the floor for high difficulty, or that the 2014 math and monsters are not well aligned in practice and don't actually work as intended (and I would agree). But it is flat incorrect to say there are not any design norms.