If a bur cuts a hole measurably larger than the head diameter, the defect is usually because of:
Runout is the more significant term clinically, because it is the primary cause of vibration during cutting and is the factor that determines the minimum diameter of the hole that may be prepared by a given bur. Burs normally cut holes measurably larger than the head diameter because of runout defects.
Concentricity
Runout
Wornout blades
Altered taper angle