system
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If the firewall is set on auto-decide mode, then rules can change. “All others auto-decide” means that the firewall will determine if these other connections are safe to be allowed. If it permits them, the application rules will be automatically modified.
Of course, rules may be different for different applications, even if they are of the same type. For instance, some might require more connections, etc. To decide, the firewall uses some sort of whitelist/blacklist, and its heuristic module.
That’s not supposed to happen. Try to repair avast (Control Panel>Add/Remove Programs>AIS>Repair)
Rules may be constantly modified, due to the firewall’s behavioral analysis of the program. Each time you re-launch the application, it will perform some heuristic checks.