Two genuinely different causes of the same rough, uneven rotation — one is purely magnetic and never goes away even unpowered, the other only exists once the drive starts switching current.
"The motor feels rough at low speed" is one of the most common complaints in motion control — and one of the easiest to misdiagnose, because two completely unrelated phenomena produce almost the same symptom. Cogging torque comes from magnets and steel wanting to snap into alignment, with no electricity involved at all. Torque ripple comes from the electronics driving the motor, and only exists while that drive is switched on. Fixing the wrong one wastes an engineering cycle without touching the actual problem.