It is statistically more likely for 2 reasons,

A) there are simply more ARR levelling dungeons, with ARR having 11 dungeons (I think) and HW and SB having 10 combined. This means that negating other factors, you still have more than 50% chance to get an ARR dungeon

B) Due to the way that the roulette works, you only get dungeons from your current level and below. This means that every single person in the queue would have access to sastasha and tam tara, and less and less people would have access to particular dungeons the further up you go. This means that you are statistically more likely to get an ARR dungeon due to more people queuing for them.

They could change the algorithm to make you more likely to get the dungeon at your appropriate level, but that wouldn't make it a roulette. Their solution is to give you equivalent XP for all dungeons, allowing you to level the same no matter the dungeon you get. And as more expansions come out, we should see slightly more variety as the % of levelling dungeons being ARR decreases