the spindle that are not synchronized to the rotational speed.

Machine structure and spindle speed resonance
  (Resonant conditions of the machine structure that are synchronized with the rotational speed).
Causing radial synchronization error (Asynchronous Error) reasons:

1 bearing preload improper (Improper preload).

2 Defective Precision bearing (Bearing defects) such as:

 Ball or roller size differences or defects (Size variation or defects in rolling elements.).

 balls or rollers and the track surface friction (Defects such as galling of rotating race).

 retainer deformation or wear poorly assembled. (bearing manufacturer cages - worn or installed improperly)

3 machine structural deformation caused by relative vibration (Structural Motion relative vibration).
4 by an external source of vibration machine vibration caused by structural deformation (Conducted vibration from the floor that caused motion between the probe and master target.).

5 machine internal vibration source (Self excited motion) caused by:

Hydraulic systems (Hydraulic system)
 Cooling system (Coolant system)
gears, belts and pulleys (Gears, belts, pulleys)
Lubrication Systems (Lubrication systems)

6 of resonant mechanical structure or spindle (Resonant frequencies of the machine elements including the spindle that are not synchronized to the rotational speed.).