Because we're talking about average amounts, it doesn't matter how many members each union has. Any difference in membership numbers wouldn't change the average.

The author is just discussing what the requested percentage raises amount to in dollars. How likely the unions are to receive the raises doesn't matter.

This strongly affirms the author's assumption, thus strengthening.. If members of the Chefs’ Union already make more than members of the Hotel Managers’ Union, and they're requesting a higher percentage raise, that certainly works out to a higher average dollar amount.

In other words, last year's raises didn't change how the average salaries of each union compare to each other. But we don't know how those numbers compared before, so this isn't useful.

Because we don't know what the average salaries were to begin with before all of these raises, this still doesn't help us compare the average salaries now. In other words, it's irrelevant.