I can suggest a reason why your tilt is off a "degree" or so. But not why. Every MacDon head I have worked on (FD70, FD75, FD1x0), has a slightly different overall travel for the sensors left and right. IIRC, it was about 0.5V difference. I could never see any reason for this---its apparently something in the frame design.
Even though your combine "calibrates" the sensors, I have noticed before that if you have a different "travel" on the left and right sensor, it often will say that you are "Off level" when you are in the middle of the range.
The other thing that really screws this up is how far the MacDon float optimizer drops when you let the feeder all the way down, way past normal operating range. This kinda screws up a good cal--which should really only cover operating range.
You might try adjusting your sensors to the same voltage when in "operating position". Adjust everything dead level on a flat surface, set your head down to 2 (or wherever you normally run) on the MacDon height indicator, and adjust both sensors equal. Get a pile of blocks or a jackstand, and block the front plate of your feeder so the float optimizer stops when the indicator hits 0 (bottom of the normal operating range.) Don't let it drop all the way down like you are disconnecting. Then recal your combine. If that does not help, its not the sensor adjustment causing your problem.
The other nice thing about doing the above is you can "double check" your system. You're out cutting and it says you are one degree off. Stop, look at the voltages in the Diagnostics. Are they close to equal or way off? Are you REALLY tilted way over, or does the combine computer just think you are?
The other thing is, any sensor has a "accuracy" spec--usually 5-10% allowable variation. I've seen as high as 20%. Slight differences can sometimes be traced to this---just the fact that sensors are not identical. I agree 1 degree seems pretty high.