The opening post assumes everyone does a five-day week with Sundays on top.
Although for those with fewer days a week, they will have longer working days.
You also get some shift patterns where you might work 10 days on the trot, and a week of rest days elsewhere in the roster.
Personally, I think Sundays should be part of the working week. But its cheaper for the railway to pay overtime than employ a proper number of staff.
In addition, when the opening post states that overtime is only paid at 100%, this will be because years ago, contracts were altered to increase basic salary, but pay overtime at that basic rate. Makes it easier for the accountants, and makes it easier for staff to apply for mortgages etc
In theory, the average should balance out that everyone gets paid the same and the company pays out the same, but staff who didn't work overtime got a massive payrise, and those who worked loads of overtime ended up taking home less.