Tldr answer, it's all dumb luck.
Longer version. Doing higher GRs only increase the xp, blood shards, and chance of getting a higher number of legendaries. The quality of the legendaries themselves isn't effected. In theory, assuming you have done a solo 70, you could get a primal ancient from a gr 10. The reason they seem to have better gear is because they are probably playing more, and doing those higher rifts faster gets you more shards etc, which gives you more chances at legendaries and in turn better odds you will get that ancient/primal you want.