It seems like lots of the payroll was being carried out by humans in local units. Humans are very good at holding lots of exceptions in our heads and, in the absence of clear rules, doing reasonable things. Computers are shit at both of the above.

So things like Bob negotiated with his boss to leave early on Thursday to cover childcare and, in exchange, is coming in early on Tuesdays... but got asked to come in Saturday and now has overtime and what does he get paid based on province or local rules? Does OT end at midnight or not? Do employees get paid non-required OT as a courtesy? And frankly, lots of payroll probably sometimes violates the rules in minor ways, but as long as the employee is happy and the org is happy no-one is upset. Computers are not good at letting small things like that go. And that's a pretty trivial case.

What about eg janitorial that is paid by multiple government orgs on some cost sharing arrangement because the orgs share office space, but is covering an event for a 3rd org in the 2nd org's space? And that janitor is on a particular contract, one of perhaps 5 or 6? If that has to get encoded into a central system, have fun with that. God only knows how many exceptions there are across a quarter of a million employees.