Queuing as healer I get Tam Tara normal about five days out of seven in a week. The other dungeons that come up the most are Toto-rak and Cutter's Cry. I assume it's mostly people leveling alts or doing the dungeons for atma books that create that pattern. I'm not a fan of any of these dungeons as a result, but in 99% of cases the run is done in less than 30 minutes so it's faster to just stick with it than it would be to leave and eat a thirty minute penalty before queuing again and possibly getting the same dungeon.