Why is defects per million $($DPM$)$ not necessarily the same as defects per million opportunities $($DPMO$)?$

DPMO accounts for variations in the production process, while DPM does not

DPMO is used for service industries, while DPM is used for manufacturing industries

DPM considers the total number of defects, while DPMO considers the number of defects per unit opportunity

DPM includes defects per unit, while DPMO includes defects per process