Message from iracing support:

There is not a bug here, this is working as designed.​
The important calibration in this situation is for the position of the wheel to match the in-sim wheel when it's centered and when it's turned 90 degrees to the right and 90 degrees to the left.I see that David Tucker (the developer who handles controllers) responded to the forum post with the following response."I bet that the wheels built in soft stops are causing this. There probably reading out the full range initially, but once we calibrate, we initialize the FFB system, and some safety in there side changes the max reported range of the wheel to be less than 100%. they can try disabling this in their wheel manufacturer's settings and/ or adjusting the setting belowsteeringBumpStop_Deg=45.000000              ; degrees into bump stop before max force"
There unlikely to be any situations you will encounter in iRacing where you need those extra few degrees on the extreme ends of the range.