What are the rankings on each job page for?
The statistical rankings on each job page are intended to provide a general idea of a job's strengths relative to the other generic jobs. However, they don't indicate the degree of difference between Job A and Job B. This page does that. The page also consolidates the rankings, allowing for easier comparison between jobs.
First off, it is important to understand the difference between a job's statistical growth and a job's statistical multipliers:
The statistical growth of a job increases a unit's "base" statistics permanently. (Base statistics consist of HP, MP, physical attack, magic attack, and speed.) The rates of statistical growth vary from job to job. For example, the rate of growth of MP is greater for a Summoner than for a Thief.
The statistical multipliers of a job, on the other hand, modify a unit's base statistics temporarily. As a result, the game returns "final" statistics for a unit. For example, if a unit who has an HP base stat of 10 changes into a Monk, which has an HP stat modifier of 5.4, his final HP value will be 54. If that same unit changes into a Black Mage, which has an HP stat modifier of 3.0, his final HP value will be 30. (The game rounds up final values if necessary.) Therefore, a unit's final statistics will depend on his current job.