There is a play on word; "It sounds great that the giant centipede was long enough to go around Mt. Mikami-yama seven and a half times, but it is just little shorter than eight rounds (in Japanese, "little shorter than eight rounds" has the same sound as hachimaki (headband)).
「三上山を7巻き半と聞けばすごいが、実は8巻き（鉢巻）にちょっと足りない」という洒落がある。