In a comment on the Forge Rat entry in my series of randomly generated monsters, Tom Fitzgerald admired the "mad, mythic underworld-style dungeon" feel that my random-generation charts tend to produce. Of course, it's deliberate; not only do I tend to look more towards Lovecraft's Dreamlands and Clark Ashton Smith's fantasies as a model for the kind of campaign I'm looking for, but also I like the approach of medieval bestiaries, fabliaux and fairy-tales, or the weird geographical notes of ancient and classical writers like Pliny. I want races of people with no heads whose faces are in their bellies, pelicans ripping out chunks of their own breast to feed her children. I don't want to restrict fantastic elements to tactical challenge instead of flavor.
What I did specifically when designing the d20 quickie tables was I split the list in half, so that 50% of the results for any roll (clustered around the middle) would be more common stuff, like the four medieval estates of warrior, peasant, tradesman/merchant, and scholar/cleric for the Professions, or vermin, herd animal, predator, and humanoid for the Monsters. The other 50% is split among ten rarer entries selected purely on the basis of what letters they start with. That's the only lip service I made towards realistic probabilities; an avian monster type is just as likely as undead, or as an amorphous ooze. There's not much concession to logic in the way the table works; I feel that applying logic after the roll produces more interesting results.