This table from
GiffyGliphy Monster Maker is gold!
Here, "Grunts" are the standard monsters, NPC soldiers, etcetera. (In contrast to Minions, Elites, etc.)
In other words, what it is saying is:
Four level 1 player characters can take on a level 4 monster as a standard encounter.
Four level 2 player characters can take on a level 6 monster as a standard encounter.
Four level 3 player characters can take on a level 8 monster as a standard encounter.
...
Four level 5 player characters can take on a level 12 monster.
Four level 9 player characters can take on a level 19 monster.
Four level 13 player characters can take on a level 22 monster.
Four level 17 player characters can take on a level 25 monster.
Because the official Challenge ratings are so inconsistent, and 5e tends to be robust, the math is flexible.
It is possible for the 2024 to systematize and standardize the monsters to accurately measure what "level" their combat traits are worth. Then separately, give a table for what counts as a standard combat encounter, depending on the level of the party and how many player characters are in the party.