Suppose we have a sample of 100 malwares and the antivirus detected 80 with heuristics set to normal. How many malwares we expect to detect with heuristics set to high? Forget about false positives because it is not the point here. Quotation form AV-Comparatives reports: "Avast, AVIRA, Kaspersky, Symantec: asked to get tested with heuristic set to high/advanced. For this reason, we recommend users to consider also setting the heuristics to high/advanced."