Programs that do backups output many messages.  Lots are "normal".
Amanda tries to keep a catalog of those that should be considered "normal".
If a version of the backup program adds new "normal" messages, or you hit
one that is not cataloged, or changes existing ones (sometimes even an
extra space or a capitalization or punctuation change), amanda will flag
it as "strange".  It is not considered a failed dump because the exit
status of the backup program indicated success.

Thanks,
Fran