Ah, I totally had missed that in 5.0 the budget is minimum and in 5.5. it is maximum! That certainly changes the things a bit. Though 5.5 encounter is still about twice as hard if both were to feature multiple monsters due the lack of the multiplier.
Yeah, that’s what I meant about them changing the XP thresholds from a floor to a ceiling. It makes it look like they shifted the difficulty benchmarks over a category, making Medium the new Easy, Hard the new Moderate, and Deadly the new Hard. But in reality, they just changed the way they were presenting the thresholds to match how most people were actually using the DMG guidelines, because the text of the 2014 DMG wasn’t clear enough about those thresholds being minimums.
Perhaps, I was only looking at the DMG rules, and if you mentioned Xanathar's previously I must have missed it.
Yeah, the Xanathar’s Guide alternate encounter building rules were their first attempt to fix the encounter balance issues in 5e without overhauling the CR system. They rebalanced the assumptions around a default of one monster per PC instead of one monster per 4.5 PCs, which yes, does make encounters with high numbers of weak enemies harder than intended, but it’s within pretty acceptable tolerances, except in the specific case of solo monsters, which would be overvalued in such a model. So, they recommended using Legendary monsters for solo fights, and picking a monster with CR a few levels above the party’s average.
The 2024 DMG encounter building guidelines, if you crunch the numbers, are basically giving the same recommendations the Xanathar’s Guide encounter building guidelines did. They’re just presenting it as an XP budget again instead of a table showing character levels on one axis and number of monsters per PC on the other, with the cross-reference telling you what CR to use. Understandably so, cause I think people found that table pretty confusing. And, instead of having a separate set of rules for using higher CR legendary monsters for solo fights, they just buffed Legendary monsters by about 40% in both damage per round and effective HP across the board.