ARAM

so i have read that ARAM goes off the champions you currently have; i have purchased all the non-promotional bundles\on sale champions, and for some reason i still get the same 5-7 champions everyday with a random one in there every once in a while (i have 40+). i also seem to be playing against the same handful of champions everyday (rarely see anything new). there are 100's of champions, i am curious why i never see them?