I got a 6th degree SOS hat off a scorpion, a level 24 sos spear off a hyugno, a 3rd degree chest off a bandit archer, and level 29 SOS chest off a hyugno. And the many many other levels I've gone through since then have yielded nothing.
This pattern you speak of is compeltely nonexistant. Yes a mangyang isn't going to drop a 64 SOS blade, but that's about it. Random she be.
o, and I grinded on the west side of the bandit den for 3 months and got different drops every single session.