Zardnaar
Legend
No need to get so bent out of shape about it. There is a reason why 5e also provides with the option for average HP per level. There is a reason why average damage exists as an option for monsters. There is a reason why proficiency bonus exists as a steady bonus for assumed proficiency in your main attack method. Regardless of the encounter subjectivity or randomness, designers still need known assumptions when designing adventures, encounters, monsters, etc.
And yet they don't follow their own rules and the classic adventures made no such assumptions.
I had an inexperienced DM who was struggling with the encounter rules. He don't really know they were on the easy side.