Yes this.
Right off the bat the 2 different devices are measuring power at different points, one’s at the crank, the other’s at the wheel. You’re gonna have drive train loss which accounts for part, there’s going to be a general difference in how they’re measured, and plenty of factors with pedal stroke / cadence / power smoothing / single sided vs dual sided when you’re in erg mode.
Just use powermatch and none of this matters as long as you tie your ftp to your stages power meter, which you should be doing anyway since that’s what you’ll use outside.