Why would anomaly detection IDSs often generate a large number of false positives?

Unfortunately, anomaly detectors and the Intrusion Detection Systems (IDS) based on them often produce a large number of false alarms, as normal patterns of user and system behavior can vary wildly. Being only able to identify correctly attacks they already know about is a characteristic of misuse detection (signature-based) IDSs. Application-based IDSs are a special subset of host-based IDSs that analyze the events transpiring within a software application. They are more vulnerable to attacks than host-based IDSs. Not being able to identify abnormal behavior would not cause false positives, since they are not identified.

Source: DUPUIS, Cl?ment, Access Control Systems and Methodology CISSP Open Study Guide, version 1.0, march 2002 (page 92).