I think for random finder to place you in a dungeon at least 1 player needs to have selected to run that dungeon. You'll see low level dungeons because people farm them for equipment (some of it sells, but some of it is for looks). Most dungeons have a 3-4 level gap between them so a person might run it 3-5 times to earn their apropriate level. There are also some dungeons which are just harder than others / not needed for equipment to 'look cool' so people avoid those as well, again less likely to find yourself sent in to one with the random finder.

Now if you have 4 level 50's in a low-level finder dungeon one would *hope* it picks at random.