Why do I keep getting the highest level dungeons for my character most of the time...As a healer, I hardly ever got anything below 61 once I leveled to 62-63. And I leveled all three healers to 70, two of which purely on leveling roulette once a day.

As for the chance of getting high level party on a low level dungeon...that's actually pretty reasonable. How many people do you think queue at once?! How many of them do you think are "newbies"?! If there are so many people that the "newbies" are already covered, then it's obvious that the ones in excess will be patched up with a really random dungeon...which may be low level. Basically, if there's no one needing a dungeon, you'll end up in one at random.