This is an old gripe. A very old one. The duty finder only subtracts from your pool based on member level and the dungeon you get, even if you have a completely full preformed party is totally random. Try being a party of all 79 level members and getting Sastasha. There are debates going way back about this and unfortunately I don't see it ever being addressed.

However, Sastasha used to be preferred if you didn't get something close to your current level due to the huge exp scaling for a very low skill dungeon but I don't know if that's still the case. Me? I que as Tank (which I'm not particularly good at) at lvl 79 thinking I'll get Brayflox or something and get Mt. Gulg. Sometimes RNG likes to get it's laughs in I guess.