Some programs are designed to rip an ISO image despite of errors on the disc, and depending on the program, it's possible that you might overlook errors during the ripping. There may be settings/options controlling if and how you are alerted of errors.
As an example it's quite possible to use DVD Decrypter in a way that continues past reading errors, and then overlook the warnings in the log.
Other programs are designed to rip an ISO image only if it can be done without errors, and you cannot overlook if that fails.
So it's difficult to answer your question without knowing which program you have used for the ripping, and perhaps some relevant settings.
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