Like Kikirio said it's pretty much an RNG (random number generated) type-thing with the loot on the bosses and his followers.
I did some testing of my own these last 2 days with Reshala on Customs and Schturman on Woods - out of 30 raids on Customs, 8 of them Reshala had his golden TT on his person and 14 raids his men had stims, nades, good ammo and armour. The other 8 raids were indifferent or had different results.
Schturman & his boys on Woods was a bit more complicated - I did 30 raids back-to-back with 100% Schturman with his boys, 5 raids he had his SVDS with ammo but no keys or backpack, his boys had their typical loadout. 12 raids Schturman had his SVDS but no ammo with a key and a stim, his boys had their typical loadout. 4 raids Schturman had his actual loadout, his boys however had completely different loadouts for what ever reason I'm still trying to work this out. The other 9 raids were totally normal.
I will be testing the other bosses in the coming days when I'm not at work or looking after my son but I think if more people looked in to this it could potentially make gameplay a little less RNG and more fun but I'm happy regardless.