Same scene photographed on FF DSLR vs crop Mirrorless, same settings: why is the mirrorless ISO so much higher?

I primarily shoot a Nikon D4 and a Fuji XT3, but rarely at the same time. I had an opportunity to photograph actor Kurt Russell at a press event this past week and had both cameras with me, and in the same light with the same settings (mostly 1/160 @ 2.8 auto ISO), the mirrorless ISO was absolutely through the roof (12,800 consistently) whereas the Nikon gave me more reasonable ISO’s, around 3200 or 4000. I’ve noticed this before and almost always grab the D4 when I’m shooting indoors in lower light, but why is the XT3 giving me such higher ISO’s? Thanks!