The flavor text is a reference to the joke: "Why is six afraid of seven?" "Because seven eight nine."
Wowhead has the drop rate of the The 5 Ring at 3% from , and the TTL itself at 0.2% from the . This suggests you have a 0.006% chance of getting The 5 Ring on any particular day; and an expected average of 16,667 daily fishing quests completed. (that's over 45 years.)
Patch 3.1.0 (14-Apr-2009): Added.