Although I have at times felt this way too, my suspicion is to say that there probably isn't any real bias. Most of the monsters are pretty common, and, unless you're very over-leveled, are near the dungeon level of Dwarftown. So it shouldn't be that much of a surprise that you often find those monsters on levels near Dwarftown. It's also a monster that you're specifically looking for. Normally you don't pay much attention to specific monsters unless they're dangerous, annoying, or out of depth, so the fact that you might have seen a dozen chaos brothers already doesn't really cross your mind.
For awhile I was convinced that the RNG took pity on players with uncontrolled teleportitis and made blink dogs more likely to spawn. I now suspect that I just pay a lot more attention to them when they do...
Hoping to win with every class, doomed. Archer, Barbarian, Bard, Beastfighter, Druid, Elementalist, Farmer, Fighter, Monk, and ULE Priest down.