The reason is that, at that time, Zenisashi was called as Shohaku method (currency exchange rate in Edo period) where 96 pieces of ichimonsen (one-mon coin) (mon is a monetary unit of old times) was 100 mon.
これは当時銭緡（ぜにさし）を省陌法と称して寛永通寳一文銭96枚の束をもって100文とする慣行からである。