Some discs will consistantly give inconsistant scan results. These discs are of a marginal quality and should be considered failures. A good disc should always scan well, baring other issues like extreme heat or faulty drive.
Discs, especially cheap ones, can have a range of issues that cause inconsistant reading problems, and not all the causes are directly related to "burn quality". On such media, lowering burn speed may resolve issues, or may not.