What I've always thought about the lore in the reference book descriptions was sort of like a Pokedex entry, where the explanation for the Digimon was more of a general thing that species does, as they are still meant to be "animals" in a way, even the humanoid Digimon. For example, Veemon's entry states that it has a prank-loving personality, but that doesn't mean there cannot be Veemon who don't like to make pranks, that's just something most Veemon like in general.
For examples of reference entries talking about how strong a specific Digimon is, I've always taken it as "this was based on a specific notable iteration of the species" because Digimon has known there can be multiple of a specific Digimon. Reference entries should be used as a sort of brief explanation of what that monster can do, or what the species does, rather than "this is the end all be all.
All of those Digimon are on the same level, it’s a Pokemon type situation where a Digimon should beat another not because he’s stronger but because he has a type advantage against it.
In this type of situation, while different Digimon may have different power levels that can prove an "Oh, a higher leveled Digimon will beat something like a rookie", but I never really saw that as "A rookie-level monster will (or must) lose against an ultimate level." Every specific member of a Digimon species can be drastically different from each other. Even the anime has shown examples like with Shoutmon being able to take out Digimon at levels way higher than him.