And yet they don't follow their own rules and the classic adventures made no such assumptions.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.
I had an inexperienced DM who was struggling with the encounter rules. He don't really know they were on the easy side.