Porky raised a question about my comments on ability scores in the post comparing experience points, ability scores, and hit dice. I was a little unfair about ability scores and hit dice; it's true that there is no correspondence between 1 ability point or 1 hit die and something in the game world (which is what I was focusing on,) but any of these things could be guessed at in the game world. The trick is to realize that they are abstract ratings.
I talked about Strength in the previous post, but let's be different and talk about Dexterity. Again, 1 point of Dexterity doesn't mean anything specific. Characters in the game world wouldn't know their Dex scores. However, someone could rank people or creatures, based on observations of how dexterous or agile they were. Take the people and creatures two at a time; is Conan more dexterous, or is The Grey Mouser? And if Grey Mouser is more dexterous, how does he compare to Joxor the Mighty? OK, how does Joxor compare to Conan? You wind up with a ranking of the three, which you can expand by comparing each new example with every individual on the list. In some cases, it might be hard to decide, but you could still, with enough examples from a wide range of people and creatures, eventually reach a list of 16 to 20 ranks of dexterity. Assign some arbitrary number to the median rank, and you could generate "Dexterity scores" for every creature.
You could do the same for each of the ability scores, or for hit dice, or for level. It's a subjective process, but it does reference things in the game, rather than meta-game knowledge.