Read it again, including that contained within the brackets.
To illustrate the "revenue thing" Captain £100,000. F/O £58,000 = £158,000.
Captain £100,000. F/O =£0 =£100,000.
Captain £100,000. F/O = Paying £25000 to sit there = £75,000.
In the latter case, F/O not only no net cost, but actually subsidises left hand seat cost.