Probably mostly true but sometimes there are legal or other reasons where tests are performed in the absence of doubt.
Edit: also the mother could have admitted to cheating but was sleeping with more than person so there could be good reason to doubt and still have positive paternity.
Or to put it another way, only 1/3 of people with reason to doubt paternity have a good reason to doubt.
Probably mostly true but sometimes there are legal or other reasons where tests are performed in the absence of doubt.
Edit: also the mother could have admitted to cheating but was sleeping with more than person so there could be good reason to doubt and still have positive paternity.