By coincidence, William, I was reading DPR's review of the X-T1 this very morning and this thread triggered a gray cell or two. DPR said:
" . . . although it's also important to understand that Fujifilm's
extremely conservative sensitivity ratings mean that ISO6400 on the X-T1 is closer to ISO4000 on most other cameras. . . ."
Source:
http://www.dpreview.com/reviews/fujifilm-x-t1/13
Frankly, I find their statement quite odd - because the ISO 2006 Standard defines a 'latitude' range of
5657 to
7127 when a camera is set to ISO 6400 and 4000 is
well outside of that range . . . about 2/3 EV below 6400 !!
Could explain your problem, though . . .
Slight rant: sometimes, when reading the 2008 Standard, I wonder if all that verbiage couldn't be simply replaced by:
"The ISO index number is whatever the manufacturer wants it to be"
If you were to swallow DPR's statement, it does give you a sorta ratio for manual compensation at higher ISO settings.