When you are an FO you are probably still writing off your training expenses, thus taking more untaxed into the pocket.
As a captain that expense has been removed/paid off and as such cannot be used each month as a tax deductible item.
Both cases are only applicable when you are on a Brookfield contract/construction with Irish company to be set up, not as a Ryanair contract as you are then an employee with basic salary and so on, taxes paid by company and no expense posts.