Good work on the 40km ITT dude!
Personally what I’ve found is when I’m running my Kickr on erg mode and I’m in small ring front and up in the 20’s on the back, it lacks some momentum turnover and therefore the pedal strokes are more taxing and could lead to the difference in FTP’s.
Tbh I’ve never tested FTP outside so I dunno how applicable what I said is.
Hopefully someone more versed in this area can shed some light.