Cursed Talisman does not abide by chance %. 10 mobs killed should yield at least 2 talismans. Total of 15 mobs yielded 0. I suspect the cursed talisman drop rate may therefore be bugged.
Cursed Talisman should have 25% drop chance in WOTLK and 80% in TBC.
I do not have a local server to check the DB right now, but with the good old boring brute force I got 47 Talismans in 200 kills which is well in the expected value I also could not find a pull request changing the drop rate and this online DB from an older version of Azerothcore has the same values listed as wowhead and wotlkdb https://wowgaming.altervista.org/aowow/?item=30157
Must have been unlucky chance for me then! But indeed, your calculations match that. Perhaps I Was unlucky!
Cheers, good one!