is it your first 951?
The interface is clunky, and I imagine some people can accidentally change calibration when trying to change the set temp.
I'd be curious what happens when you do a factory reset. If it significantly improves it, I would suspect user error. Maybe not yours, if the iron has ever been used by anyone else. At any rate, you can calibrate the station to w/e you want, although it will vary a bit, tip to tip. And there may be some offsets in the overall range.
So.... if you found yourself limited by the 840F max set temp, you could turn it up to 11, if you wanted. I guess your iron is already calibrated to the Spinal Tap standard.
I have tested several irons. 4x 888's and a couple of T12 irons. All my hakko stuff was within maybe +-15 C, or say 30F, depending on the tip (and most of that was due to variation between handpieces when swapped around). One of the clones was maybe 30C higher than the displayed set temp. I guess that's pretty close to 50F. The Bakon clone was actually pretty close, IIRC. 10C over, tops. This is more impressive because the manual states something to this effect. To expect the actual temp to be 10C higher than the set temp, and it was pretty much spot on.