Because Tsumabuki was Satou-san's 13th 'lover' (before him was 1 girl and 12 guys) so that makes him the Lucky Number 13 that ended up with cute little Satou!
Ah-ha__ I see,, wow you're really observant person,, thank you dear ^__^"
Oh thank you for your question and answer! Im going to ask the same thing.
Ratt! Thanks you for enlightening us! I feel so happy I undestand the title now :D
Wow this manga really so much fun to read. Though i feel so sorry for his bad luck, but i couldn't to stopped laughing..
But why the tittle it was " Lucky Number 13" ?