the 7th could be more popular due to violence being a worse face-value attribute to be surrounded by than fraudulence. of course, disreagarding that the level of torture is correlated with depth.

anecdotally, this was a point of confusion in a humanities course for some of my peers, as many would argue violent crimes to be the worst.

Another theory is that it borrows from Seven Heavens.¹

¹ https://en.wikipedia.org/wiki/Seven_heavens