what is the difference between max nose wheel steering angle and effective steering angle?
When the two sides of the main landing gear move, each one tends to force a circle with its own radius and center, and is at the same time influenced by the nose landing gear own radius of turn.


Source: A350-900 Aircraft characteristics, airport and maintenance planning

Some tire sliding must occur due to the difference in circles, the actual center is between the three individual centers mentioned and the radius R3 is therefore a bit impacted.

Depending on the differential action on the brakes, and the use of engine thrust asymmetry, the result varies by a few percents (between types of turn 1 and 2 above).