strange/different mminfo output on different days

mminfo -s server -xc; -t "one month ago" -q "valid,!incomplete" -r client,name,ssid,ssflags,sscreate,level,totalsize -o cnt >test.txt

but i think that if the tapes are overwritten then i loose the information so the behaviour is expected. am i right?

Could you please paste the mminfo command you are running to get reports ?

Yes. When the tape is recycled/relabeled then the info about backups on that tape is deleted from mediadb. So generally if you run mminfo on day X and then 5 days later you run again the same mminfo command, but between there ware tapes recycle/relabel operations, your results will be different.