For the latter, that works too. Expert Roulette currently works the same way, after all.
For the former, it's more about equalizing the amount of dungeons available to each roulette. If we don't do the step of reshuffling the dungeons across both roulettes but equalize both, some people will just not bother doing Leveling Roulette at all (and switching to exclusively doing 50/60/70/80 roulette) due to a big chunk of the dungeons consisting of pre-Lv. 50 ARR dungeons. Then we have the problem of progression being slowed for new players. With the roulettes being re-categorized, the story roulette actually contains more higher level content, while the optional roulette has more mid-game/expansion level cap content.