Farming Color of Lumina Question

[Ereius]

I am currently farming Clair obscure boss, today i noticed that somehow i was getting double rewards for killing the said boss. Where usually i get 5 color of lumina, 30% of the time if was getting 10, + double xp and double gold. I never noticed this before. Is something triggering this double reward or is it just random? (no is not the 20% no damage thingy)

[Ratsplat]

The only bonus xp I've seen outside of the 20% is from the dancers who create clones. So unless clones are being created, I'm unsure.