It is likely caused by drill runout during machining. 

Possible Causes

① Runout when mounting the drill

　Countermeasure: Keep mounting runout within 20 μm (0.020 mm).

② Large lip height variation after regrinding causes drill tip wobble during machining.

　Countermeasure: Review the regrinding process.

③ Drill wobbling at the hole entrance

　Countermeasure: Increase feed per revolution to reduce wobbling (suppression by thrust force).

Resulting defect: Poor roundness